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regcomp.c: Skip work that is a no-op
There are a few characters in the Latin1 range that can be folded to by
above-Latin1 characters. Some of these are folded to as part of a
single character fold, like KELVIN SIGN folds to 'k'. More are folded
to as part of a multi-character fold. Until this commit, there wasn't a
quick way to distinguish between the two classes. A couple of places
only want the single-character ones. It is more efficient to look for
just those than to include the multi-char ones which end up not doing
anything. This uses a bit in l1_char_class_tab.h to indicate those
characters that are in the desired class.