Fix bug introduced into indexable_outerrelids() by an ill-considered

Fix bug introduced into indexable_outerrelids() by an ill-considered
"optimization".  When we find a potentially useful joinclause, we
have to add all its other required_relids to the result, not only the
other clause_relids.  They are different in the case of a joinclause
whose applicability has to be postponed due to outer join.  We have
to include the extra rels because otherwise, after best_inner_indexscan
masks the join rels with index_outer_relids, it will always fail to
find the joinclause as applicable.  Per report from Husam Tomeh.