Improve code reuse between trans/_match.rs and check_match.rs

I believe there's more commonality to be found there but maybe small steps are better. I'm also in the process of documenting what I can, I will see if I can add it to this PR.

It also seems to me that ideally some of this stuff (especially the pattern sanity check) could live as a separate compiler-agnostic module but I understand this may not be the right time (if not the worst) to start the process of modularising rustc.
Please alpha-rename this type item to `enter_pats` (plural), and update the corresponding use in `enter_match`.