[Support] Fix alternation support in backreferences (PR60073)

backref() always performs a full match on the remaining string,
and as such also needs to be matched against the whole remaining
strip. For alternations, the match was performed against just the
sub-strip for one alternative, which would of course fail to match
the whole string.

This can be done by skipping the part of the strip between OOR1
and O_CH, so that only the first alternative in the strip is
matched, and the remaining ones are skipped. Indeed, the necessary
OOR1 skipping code was already implemented in the easy-path of
backref(), so this is clearly how it was supposed to work.

However, there were two bugs: First, under this scheme we should
be passing the stop point of the original strip, not just the
alternative sub-strip. Second, while skipping for OOR1 was
implemented, handling for O_CH was missing. This would occur when
the last alternative matches, as O_CH is preceded by an implicit
OOR1 only.

Fixes https://github.com/llvm/llvm-project/issues/60073.