Keep ref-returning switches as statements

C# has no ref-returning switch expression. Skip the transform for StackType.Ref and cover the statement form in RefLocalsAndReturns.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0dd407b6-9410-48df-add5-761ca4a8dec0

ICSharpCode.Decompiler.Tests/TestCases/Pretty/RefLocalsAndReturns.cs

@@ -282,6 +282,19 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return ref DefaultInt;
 		}
 
+		public static ref int SwitchRefReturn(int index)
+		{
+			switch (index)
+			{
+				case 0:
+					return ref numbers[0];
+				case 1:
+					return ref numbers[1];
+				default:
+					throw new InvalidOperationException();
+			}
+		}
+
 		public static ref int LastOrDefault()
 		{
 			if (numbers.Length != 0)

ICSharpCode.Decompiler/IL/Transforms/ExpressionTransforms.cs

@@ -676,6 +676,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			// Exactly one of resultVariable/leaveTarget must be null
 			if ((resultVariable == null) == (leaveTarget == null))
 				return;
+			if (resultType == StackType.Ref)
+				return;
 			if (switchInst.Value is StringToInt str2int)
 			{
 				// validate that each integer is used for exactly one value