movign assert and giving more clear message

Author: weinbe58

src/kirin/dialects/ilist/rewrite/flatten_add.py

@@ -16,6 +16,9 @@ def rewrite_Statement(self, node: ir.Statement) -> RewriteResult:
         ):
             return RewriteResult()
 
+        assert isinstance(rhs_type, types.Generic), "Expecting generic type for IList"
+        assert isinstance(lhs_type, types.Generic), "Expecting generic type for IList"
+
         # check if we are adding two ilist.New objects
         new_data = ()
 
@@ -48,9 +51,6 @@ def rewrite_Statement(self, node: ir.Statement) -> RewriteResult:
         ):
             return RewriteResult()
 
-        assert isinstance(rhs_type := rhs.type, types.Generic), "Impossible"
-        assert isinstance(lhs_type := lhs.type, types.Generic), "Impossible"
-
         lhs_elem_type = lhs_type.vars[0]
         rhs_elem_type = rhs_type.vars[0]