Fix #937: support implicit Ref->I4 conversions

These are not allowed in Ecma-335 Table 8: Conversion Operations,
but the .NET runtime accepts them anyways, and C++/CLI generates them.

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -1056,10 +1056,24 @@ namespace ICSharpCode.Decompiler.IL
 				} else if (expectedType == StackType.I4 && inst.ResultType == StackType.I) {
 					// C++/CLI also sometimes implicitly converts in the other direction:
 					inst = new Conv(inst, PrimitiveType.I4, false, Sign.None);
-				} else if (expectedType == StackType.I && inst.ResultType == StackType.Ref) {
+				} else if (inst.ResultType == StackType.Ref) {
 					// Implicitly stop GC tracking; this occurs when passing the result of 'ldloca' or 'ldsflda'
 					// to a method expecting a native pointer.
 					inst = new Conv(inst, PrimitiveType.I, false, Sign.None);
+					switch (expectedType) {
+						case StackType.I4:
+							inst = new Conv(inst, PrimitiveType.I4, false, Sign.None);
+							break;
+						case StackType.I:
+							break;
+						case StackType.I8:
+							inst = new Conv(inst, PrimitiveType.I8, false, Sign.None);
+							break;
+						default:
+							Warn($"Expected {expectedType}, but got {StackType.Ref}");
+							inst = new Conv(inst, expectedType.ToKnownTypeCode().ToPrimitiveType(), false, Sign.None);
+							break;
+					}
 				} else if (expectedType == StackType.Ref) {
 					// implicitly start GC tracking / object to interior
 					if (!inst.ResultType.IsIntegerType() && inst.ResultType != StackType.O) {