regcomp.c: Relax some restrictions on optimizations for locale

regcomp.c: Relax some restrictions on optimizations for locale

Prior to this commit, we didn't do any inversions for bracketed
character classes running under locale.  However, this is more strict
than necessary.  If there is no folding, and everything else is known at
compile time, then what is matched when the result is complemented is
well-defined, and can be done now.  (Also clarifies one of the affected
comments)