[xamlc] add a new ResourceLoader.IsEnabled check (#11754)

Context: https://github.com/jonathanpeppers/Xamarin.Forms.Benchmarks

I noticed a C# binding by hand is still a bit faster than bindings
written in Xaml in various forms:

|         Method |     Mean |     Error |    StdDev |  Gen 0 |  Gen 1 | Gen 2 | Allocated |
|--------------- |---------:|----------:|----------:|-------:|-------:|------:|----------:|
|  ByHandOneTime | 3.210 us | 0.0836 us | 0.0553 us | 0.5531 | 0.0038 |     - |   3.42 KB |
|   TypedOneTime | 4.087 us | 0.0560 us | 0.0370 us | 0.8163 | 0.0076 |     - |   5.05 KB |
| RegularOneTime | 5.375 us | 0.0117 us | 0.0077 us | 0.9079 | 0.0076 |     - |   5.61 KB |
|         ByHand | 5.552 us | 0.6508 us | 0.4305 us | 0.5951 | 0.1450 |     - |   3.71 KB |
|          Typed | 6.992 us | 0.4679 us | 0.3095 us | 0.9155 | 0.3052 |     - |   5.67 KB |
|        Regular | 7.822 us | 0.3988 us | 0.2638 us | 0.9460 | 0.3128 |     - |   5.86 KB |

Looking at the code that XamlC generates for any `Page` or `View`
created from Xaml:

    private void InitializeComponent()
    {
        if (ResourceLoader.CanProvideContentFor(new ResourceLoader.ResourceLoadingQuery
        {
            AssemblyName = typeof(BindingLabel).GetTypeInfo().Assembly.GetName(),
            ResourcePath = "Views/BindingLabel.xaml.xaml",
            Instance = this
        }))
        {
            __InitComponentRuntime();
            return;
        }
        //...

This is produced for a simple view such as:

    <?xml version="1.0" encoding="UTF-8"?>
    <Label
        xmlns="http://xamarin.com/schemas/2014/forms"
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
        x:Class="Xamarin.Forms.Benchmarks.BindingLabel"
        Text="{Binding Text}"
    />

The `ResourceLoader` check was originally for the Xamarin.Forms
previewer -- and is now used by Hot Reload.

The call that seems relatively expensive here:

    typeof(BindingLabel).GetTypeInfo().Assembly.GetName()

If you created N `BindingLabel` objects, this would get called N
times. A complex Xamarin.Forms app would potentially have a lot of
Xaml views & pages like this.

If we added a new `ResourceLoader.IsEnabled` property, we could make
the check instead do:

    if (ResourceLoader.IsEnabled && ResourceLoader.CanProvideContentFor(...))

This skips the `Assembly.GetName()` call completely now.

~~ Results ~~

Using this benchmark:

https://github.com/jonathanpeppers/Xamarin.Forms.Benchmarks/blob/xamarin.forms-resourceloader/Xamarin.Forms.Benchmarks/Benchmarks/Bindings.cs

|   Method |     Mean |     Error |    StdDev |   Median | Gen 0 | Gen 1 | Gen 2 | Allocated |
|--------- |---------:|----------:|----------:|---------:|------:|------:|------:|----------:|
|   ByHand | 3.546 us | 0.0565 us | 0.1613 us | 3.502 us |     - |     - |     - |   3.76 KB |
| Regular2 | 4.542 us | 0.0696 us | 0.1985 us | 4.541 us |     - |     - |     - |   4.88 KB |
|  Regular | 6.041 us | 0.0531 us | 0.1490 us | 6.065 us |     - |     - |     - |   5.92 KB |

`Regular2` has an equivalent of the changes here and is a bit closer
to being on par with the "by hand" binding.

If I time the startup time for this page on Android:

    Before:
    08-11 15:27:34.005  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +679ms
    08-11 15:27:37.797  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +648ms
    08-11 15:27:41.551  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +622ms
    08-11 15:27:45.436  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +649ms
    08-11 15:27:49.234  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +648ms
    08-11 15:27:53.048  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +633ms
    08-11 15:27:56.864  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +635ms
    08-11 15:28:00.686  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +629ms
    08-11 15:28:04.466  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +634ms
    08-11 15:28:08.265  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +636ms
    Average(ms): 641.3
    Std Err(ms): 5.03774640974403
    Std Dev(ms): 15.930752929127

    After:
    08-11 15:30:22.575  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +644ms
    08-11 15:30:26.341  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +614ms
    08-11 15:30:30.126  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +624ms
    08-11 15:30:33.907  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +626ms
    08-11 15:30:37.708  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +621ms
    08-11 15:30:41.493  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +616ms
    08-11 15:30:45.290  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +636ms
    08-11 15:30:49.157  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +623ms
    08-11 15:30:52.955  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +632ms
    08-11 15:30:56.738  1391  1448 I ActivityTaskManager: Displayed com.xamarin.forms.helloforms/crc6450e568c951913723.MainActivity: +627ms
    Average(ms): 626.3
    Std Err(ms): 2.8715075405709
    Std Dev(ms): 9.08050414655241

Using the project here on a Pixel 3 XL:

https://github.com/jonathanpeppers/Xamarin.Forms/tree/resourceloader-benchmark/HelloForms/HelloForms

Author: jonathanpeppers

Xamarin.Forms.Build.Tasks/XamlCTask.cs

@@ -282,6 +282,10 @@ bool TryCoreCompile(MethodDefinition initComp, MethodDefinition initCompRuntime,
 					//First using the ResourceLoader
 					var nop = Instruction.Create(Nop);
 
+					// if (ResourceLoader.IsEnabled && ...
+					il.Emit(Call, module.ImportPropertyGetterReference(("Xamarin.Forms.Core", "Xamarin.Forms.Internals", "ResourceLoader"), "IsEnabled", isStatic: true));
+					il.Emit(Brfalse, nop);
+
 					il.Emit(Newobj, module.ImportCtorReference(("Xamarin.Forms.Core", "Xamarin.Forms.Internals", "ResourceLoader/ResourceLoadingQuery"), 0));
 
 					//AssemblyName

Xamarin.Forms.Core/Internals/ResourceLoader.cs

@@ -34,12 +34,11 @@ internal set {
 
 		[Obsolete("Can't touch this")]
 		[EditorBrowsable(EditorBrowsableState.Never)]
-		public static bool CanProvideContentFor(ResourceLoadingQuery rlq)
-		{
-			if (_resourceProvider2 == null)
-				return false;
-			return _resourceProvider2(rlq) != null;
-		}
+		public static bool IsEnabled => _resourceProvider2 != null;
+
+		[Obsolete("Can't touch this")]
+		[EditorBrowsable(EditorBrowsableState.Never)]
+		public static bool CanProvideContentFor(ResourceLoadingQuery rlq) => _resourceProvider2?.Invoke(rlq) != null;
 
 		public class ResourceLoadingQuery
 		{