Check for mutable borrow of counter variable

tests/compile-fail/for_loop.rs

@@ -179,4 +179,8 @@ fn main() {
     let mut _index = 1;
     if false { _index = 0 };
     for _v in &vec { _index += 1 }
+
+    let mut _index = 0;
+    { let mut _x = &mut _index; }
+    for _v in &vec { _index += 1 }
 }