Relax ExpressionBuilder invariant for lifted instructions with unknown result type.

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -166,6 +166,8 @@ namespace ICSharpCode.Decompiler.CSharp
 					Debug.Assert(cexpr.Type.GetSign() != Sign.None, "Must have a sign specified for zero/sign-extension");
 				}
 				else if (inst is ILiftableInstruction liftable && liftable.IsLifted)
+				{
+					if (liftable.UnderlyingResultType != StackType.Unknown)
 					{
 						Debug.Assert(NullableType.IsNullable(cexpr.Type));
 						IType underlying = NullableType.GetUnderlyingType(cexpr.Type);
@@ -179,6 +181,7 @@ namespace ICSharpCode.Decompiler.CSharp
 							Debug.Assert(underlying.GetStackType() == liftable.UnderlyingResultType);
 						}
 					}
+				}
 				else if (inst.ResultType == StackType.Ref)
 				{
 					Debug.Assert(cexpr.Type.GetStackType() == StackType.Ref || cexpr.Type.GetStackType().IsIntegerType());