Relax ExpressionBuilder invariant for lifted instructions with unknown result type.

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -167,16 +167,19 @@ namespace ICSharpCode.Decompiler.CSharp
 				}
 				else if (inst is ILiftableInstruction liftable && liftable.IsLifted)
 				{
-					Debug.Assert(NullableType.IsNullable(cexpr.Type));
-					IType underlying = NullableType.GetUnderlyingType(cexpr.Type);
-					if (liftable.UnderlyingResultType.IsIntegerType())
+					if (liftable.UnderlyingResultType != StackType.Unknown)
 					{
-						Debug.Assert(underlying.GetStackType().IsIntegerType(), "IL instructions of integer type must convert into C# expressions of integer type");
-						Debug.Assert(underlying.GetSign() != Sign.None, "Must have a sign specified for zero/sign-extension");
-					}
-					else
-					{
-						Debug.Assert(underlying.GetStackType() == liftable.UnderlyingResultType);
+						Debug.Assert(NullableType.IsNullable(cexpr.Type));
+						IType underlying = NullableType.GetUnderlyingType(cexpr.Type);
+						if (liftable.UnderlyingResultType.IsIntegerType())
+						{
+							Debug.Assert(underlying.GetStackType().IsIntegerType(), "IL instructions of integer type must convert into C# expressions of integer type");
+							Debug.Assert(underlying.GetSign() != Sign.None, "Must have a sign specified for zero/sign-extension");
+						}
+						else
+						{
+							Debug.Assert(underlying.GetStackType() == liftable.UnderlyingResultType);
+						}
 					}
 				}
 				else if (inst.ResultType == StackType.Ref)