Fix "for ... if" bug introduced since 3.6.2...

However we've isolated and documented the 3.x grammar a little bit better

uncompyle6/parsers/parse37.py

@@ -952,6 +952,8 @@ class Python37Parser(Python37BaseParser):
         """
         stmt               ::= if_expr_lambda
         stmt               ::= conditional_not_lambda
+
+        # If statement inside a loop:
         stmt               ::= ifstmtl
 
         if_expr_lambda     ::= expr jmp_false expr return_if_lambda