regex-emacs.c (mutually_exclusive_aux): Rework again

regex-emacs.c (mutually_exclusive_aux): Rework again

Rework the way we handle loops.  This new code does not really work
better than the previous one, but it has the advantage of being "fail
safe" and also that we can dynamically check if our assumptions about
the shape of the bytecode are satisfied or not.

* src/regex-emacs.c (mutually_exclusive_aux): Replace `done_beg` and
`done_end` with `loop_beg` and `loop_end`.
(mutually_exclusive_p): Adjust accordingly.
(analyze_first): Fix incorrect assertion.