fix[codegen]: fix gas usage of iterators

cd31867 introduces a gas regression, which is that
`IRnode.is_literal` evaluates to true even for pointers (which are not
source-level literals). this commit changes the condition to be
`not .is_pointer`, which should be the correct condition for needing to
strictify into memory.

vyper/codegen/ir_node.py

@@ -425,8 +425,6 @@ def int_value(self) -> int:
 
     @property
     def is_pointer(self) -> bool:
-        # not used yet but should help refactor/clarify downstream code
-        # eventually
         return self.location is not None
 
     @property  # probably could be cached_property but be paranoid

vyper/codegen/stmt.py

@@ -257,8 +257,8 @@ def _parse_For_list(self):
 
         ret = ["seq"]
 
-        # list literal, force it to memory first
-        if iter_list.is_literal:
+        # if it's a list literal, force it to memory first
+        if not iter_list.is_pointer:
             tmp_list = self.context.new_internal_variable(iter_list.typ)
             ret.append(make_setter(tmp_list, iter_list))
             iter_list = tmp_list