regcomp.c: Update comment to new reality
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.