Update for_helpers.py

Author: BobTheBuidler

mypyc/irbuild/for_helpers.py

@@ -470,10 +470,11 @@ def make_for_loop_generator(
                 else:
                     start_reg = builder.accept(expr.args[0])
                     end_reg = builder.accept(expr.args[1])
-                    step = 1 if num_args == 2 else builder.extract_int(expr.args[2])
+                    step_ = 1 if num_args == 2 else builder.extract_int(expr.args[2])
                     assert isinstance(
-                        step, int
+                        step_, int
                     ), "this was validated above, the assert is for mypy"
+                    step = step_
 
                 for_range = ForRange(builder, index, body_block, loop_exit, line, nested)
                 for_range.init(start_reg, end_reg, step)