regex.c: Fix recent regression with mutually_exclusive_p

regex.c: Fix recent regression with mutually_exclusive_p

The new analysis code ended up increasing the scope of an optimization
a bit too far.  Reign it in.

* src/regex-emacs.c (struct mutexcl_data): Add `unconstrained` field.
(mutually_exclusive_one): Use and set it.
(mutually_exclusive_p): Initialize it.

* test/src/regex-emacs-tests.el (regexp-tests-backtrack-optimization):
Add test.