regexec.c: Add function to compare /i Latin1, pre-folded

regexec.c: Add function to compare /i Latin1, pre-folded

This adds a function like Perl_foldEQ_latin1(), but the second string is
assumed to be pre-folded.  I made it a static function in the only file
that uses it, regexec.c, to take advantage of the C optimizer.

More traditionally would be to make a new API function which takes a
flags parameter.  But the whole purpose of this is speed, and so I chose
not to go that route.