integrate #18349 from maint-5.8:

integrate #18349 from maint-5.8:
At least partially address [perl #10000] by speeding
up both the ASCII case (by about 2-3%) and the UTF-8 case
(by about 45%).  The major trick is to avoid hitting the
costly S_reginclass().  (Even before this patch the speedup
since 5.8.0 was about 40-50%.)  After this the UTF-8 case is
still about 30-60% slower than the ASCII case.  (Note that
I'm unable to reproduce the 10-fold speed difference of the
original bug report; I can see a factor of 2 or 3, but no more.)

p4raw-id: //depot/perl@18529
p4raw-integrated: from //depot/maint-5.8/perl@18528 'merge in'
	regexec.c (@18347..)