Dropping disjoint from an OR may yield incorrect results, as some
analysis may have converted it to an Add implicitly (e.g. SCEV used
for dependence analysis). Instead, replace it with an equivalent Add.

This is possible as all users of the disjoint OR only access lanes
where the operands are disjoint or poison otherwise.

Note that replacing all disjoint ORs with ADDs instead of dropping the
flags is not strictly necessary. It is only needed for disjoint ORs
that SCEV treated as ADDs, but those are not tracked.

…t-or-with-add