regcomp.c: Better optimize [classes] under /aa.

regcomp.c: Better optimize [classes] under /aa.

An optimization introduced in 5.14 is for bracketed character classes of
the very special form like [Bb].  These can be optimized into an
EXACTFish node.  In this case, they can be optimized to an EXACTFA node
since they are ASCII characters.  If the surrounding options are /aa, it
is likely that any adjacent EXACTFish nodes will be EXACTFA, so optimize
to that node instead of the previous EXACTFU.  This will allow the
optimizer to collapse any adjacent nodes.  For example

    qr/a[B]c/aai

will now get optimized to  an EXACTFA of "abc".  Previously it would
have gotten optimized to EXACTFA<a> . EXACTFU<b> . EXACTFA<c>.