regcomp.c: optimization for qr/[...]/il

regcomp.c: optimization for qr/[...]/il

Certain matches are calculated as being legal only when the current
execution time local is a UTF-8 one.  However, a character class can
have multiple components (and usually does), and some of those components
may be duplicates of some of these matches, and be valid regardless of
the locale.  This commit removes them from the tentative list, and if it
goes to zero, clears it.  This will improve execution time slightly.