regex.c: Consolidate the two analysis functions

We currently have two functions that analyze the bytecode
to try and apply optimizations: `analyze_first` and `mutually_exclusive_p`.
Extract the common code between them into a new function `forall_firstchar`,
and then rewrite the old ones on top of that one.

Along the way, we get slightly better analyses that reverts
the recent de-optimizations but without re-introducing the
corresponding bugs.

* src/regex-emacs.c (forall_firstchar_1, forall_firstchar): New functions.
(analyze_first_old): Rename from `analyze_first`.
(struct anafirst_data): New struct.
(analyze_first_fastmap, analyze_first_null): New functions.
(analyze_first): Rewrite to use `forall_firstchar` with those two functions.
Take a `bufp` rather than a `multibyte` arg.
(regex_compile, re_compile_fastmap): Adjust calls accordingly.
(struct mutexcl_data): New struct.
(mutually_exclusive_one): New function.
(mutually_exclusive_p): Rewrite to use `forall_firstchar` with that function.

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