Tweak regex source gen for single char repeater (#118137)

This doesn't help with perf, it just makes the generated code a bit cleaner. Instead of e.g.
```C#
if ((uint)slice.Length < 20)
{
    return false; // The input didn't match.
}

if (slice.Slice(0, 20).ContainsAnyExceptInRange('a', 'c'))
{
    return false; // The input didn't match.
}
```
we'll now get:
```C#
if ((uint)slice.Length < 20 || slice.Slice(0, 20).ContainsAnyExceptInRange('a', 'c'))
{
    return false; // The input didn't match.
}
```

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -4181,13 +4181,6 @@ void EmitSingleCharRepeater(RegexNode node, bool emitLengthCheck = true)
                 }
                 else
                 {
-                    // if ((uint)(sliceStaticPos + iterations - 1) >= (uint)slice.Length) goto doneLabel;
-                    if (emitLengthCheck)
-                    {
-                        EmitSpanLengthCheck(iterations);
-                        writer.WriteLine();
-                    }
-
                     // If we're able to vectorize the search, do so. Otherwise, fall back to a loop.
                     // For the loop, we're validating that each char matches the target node.
                     // For Contains{Any}, we're looking for the first thing that _doesn't_ match the target node,
@@ -4196,13 +4189,26 @@ void EmitSingleCharRepeater(RegexNode node, bool emitLengthCheck = true)
                     {
                         string containsExpr = indexOfExpr.Replace("IndexOf", "Contains");
 
-                        using (EmitBlock(writer, $"if ({sliceSpan}.Slice({sliceStaticPos}, {iterations}).{containsExpr})"))
+                        string condition = $"{sliceSpan}.Slice({sliceStaticPos}, {iterations}).{containsExpr}";
+                        if (emitLengthCheck)
+                        {
+                            condition = $"{SpanLengthCheck(iterations)} || {condition}";
+                        }
+
+                        using (EmitBlock(writer, $"if ({condition})"))
                         {
                             Goto(doneLabel);
                         }
                     }
                     else
                     {
+                        // if ((uint)(sliceStaticPos + iterations - 1) >= (uint)slice.Length) goto doneLabel;
+                        if (emitLengthCheck)
+                        {
+                            EmitSpanLengthCheck(iterations);
+                            writer.WriteLine();
+                        }
+
                         string repeaterSpan = "repeaterSlice"; // As this repeater doesn't wrap arbitrary node emits, this shouldn't conflict with anything
                         writer.WriteLine($"ReadOnlySpan<char> {repeaterSpan} = {sliceSpan}.Slice({sliceStaticPos}, {iterations});");