Avoid relying on subgoals order for refine
This is in preparation for https://github.com/coq/coq/pull/7825
It has already been discussed some time ago here: https://github.com/AbsInt/CompCert/pull/325
I'm happy to test compatibility with older Coq versions, but I couldn't find which versions of Coq are officially supported by the Menhir Coq backend.