regcomp.c: Trade a little time for simplicity

regcomp.c: Trade a little time for simplicity

Perl currently has two sets of inversion lists for the POSIX classes
built in.  One set is for the entire Unicode range; the other for just
the ASCII range.  This latter set could be derived from the larger one
at run time by doing an intersection with ASCII.  This commit starts the
process of entirely removing the second set, thus avoiding its
bookkeeping.  This commit avoids one use of the ASCII set, instead
adding an intersection with ASCII after all the POSIX classes in a
bracketed character set have been combined.  Thus the penalty is one
intersection per compilation of each bracketed character classes that
contain POSIX classes.