regcomp.c: Move some locale initialization code

regcomp.c: Move some locale initialization code

This moves some code down a few lines, rewording the comments, and doing
some things only once, instead of each time through the loop.

This eliminates some tests, as the new place already has determined that
some of the preconditions for executing the code have been met.  It
keeps us from generating unnecessary locale nodes for platforms which
don't have isascii().