[dart2wasm] Inline Pointer constructor

Currently none of dart2wasm or wasm-opt inline `Pointer._`, resulting in
bad code generation when the allocated pointers are directly unboxed.
Example:

```
import 'dart:ffi';

@Native<Pointer<Int64> Function()>()
external Pointer<Int64> intPtr();

@Native<Int64 Function(Pointer<Int64>)>()
external int derefIntPtr(Pointer<Int64> ptr);

void main() {
  Pointer<Int64> ptr = intPtr();
  ptr += 1;
  int i = derefIntPtr(ptr);
  print(i);
}
```

Current output:

```
(func $main (;299;)
  i32.const 83
  call $ffi.intPtr (import)
  call $Pointer._
  struct.get $_Type $field2
  i32.const 8
  i32.add
  call $Pointer._
  struct.get $_Type $field2
  call $ffi.derefIntPtr (import)
  struct.new $BoxedInt
  call $print)
```

With this CL the `Pointer` allocation disappears:

```
(func $main (;299;)
  i32.const 83
  call $ffi.intPtr (import)
  i32.const 8
  i32.add
  call $ffi.derefIntPtr (import)
  struct.new $BoxedInt
  call $print)
```

Change-Id: I54a2fd37bc9e8ab26c6394e0b8175d65ecf64cb4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/399561
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

sdk/lib/_internal/wasm/lib/ffi_patch.dart

@@ -10,6 +10,7 @@ final class Pointer<T extends NativeType> {
   @pragma("wasm:entry-point")
   WasmI32 _address;
 
+  @pragma("wasm:prefer-inline")
   Pointer._(this._address);
 
   @patch