chore: fix loop variable typing (#3571)

there is an inconsistency between codegen and typechecking types when a
loop iterates over a literal list.

in this code, although it compiles, during typechecking, `i` is typed as
a `uint8` while `[1,2,3]` is annotated with `int8[3]`

```
@external
def foo():
for i in [1,2,3]:
    a: uint8 = i
```

since the iterator type is always correct, this commit is a chore since
it fixes the discrepancy, but there is no known way to "abuse" the
behavior to get a wrong codegen type.

chainsec june 2023 review 5.15

vyper/codegen/stmt.py

@@ -316,10 +316,8 @@ def _parse_For_list(self):
         with self.context.range_scope():
             iter_list = Expr(self.stmt.iter, self.context).ir_node
 
-        # override with type inferred at typechecking time
-        # TODO investigate why stmt.target.type != stmt.iter.type.value_type
         target_type = self.stmt.target._metadata["type"]
-        iter_list.typ.value_type = target_type
+        assert target_type == iter_list.typ.value_type
 
         # user-supplied name for loop variable
         varname = self.stmt.target.id

vyper/semantics/analysis/annotation.py

@@ -85,14 +85,14 @@ def visit_Return(self, node):
     def visit_For(self, node):
         if isinstance(node.iter, (vy_ast.Name, vy_ast.Attribute)):
             self.expr_visitor.visit(node.iter)
-        # typecheck list literal as static array
+
+        iter_type = node.target._metadata["type"]
         if isinstance(node.iter, vy_ast.List):
-            value_type = get_common_types(*node.iter.elements).pop()
+            # typecheck list literal as static array
             len_ = len(node.iter.elements)
-            self.expr_visitor.visit(node.iter, SArrayT(value_type, len_))
+            self.expr_visitor.visit(node.iter, SArrayT(iter_type, len_))
 
         if isinstance(node.iter, vy_ast.Call) and node.iter.func.id == "range":
-            iter_type = node.target._metadata["type"]
             for a in node.iter.args:
                 self.expr_visitor.visit(a, iter_type)
             for a in node.iter.keywords: