Ensure set_ctrl(n, c) keeps the loop membership list
(_body) up to date. It should be the case that
1) get_loop(get_ctrl(loop->_body.at(i))) == loop
2) if get_loop(get_ctrl(n))->is_inner loop() then
   _body.at(i) == n for some i

Ensure this after clone loop (and before partial peeling does it's thing.)