Change method of passing some info from regcomp to regexec

Change method of passing some info from regcomp to regexec

For the last several releases, the fact that an ANYOF node could match
something outside its bitmap has been passed to regexec.c by having its
ARG field not be -1 (appropriately cast).  A bit was set if the match
could occur even if the target string was not UTF-8 encoded.  This
design was used to save a bit, as previously there was a bit also for it
matching UTF-8 strings.

That design is no longer tenable, as a future commit will have a third
(independent) reason for something to match outside the bitmap,  This
commits uses the current spare bit flag to indicate if the match can
only occur if the target string is UTF-8.