* Define a new private op constant OPpITER_REFALIAS
* Presence of this flag on OP_ITER means the iteration var is a
refalias
* Throw away the LVREF/REFGEN part of the optree in favour of just
setting this flag
This change is useful on its own, and also a good stepping-stone towards
being able to implement multiple refalias variables in foreach; as per
https://github.com/Perl/perl5/issues/24027