Improve pull_up_subqueries logic so that it doesn't insert unnecessary

Improve pull_up_subqueries logic so that it doesn't insert unnecessary
PlaceHolderVar nodes in join quals appearing in or below the lowest
outer join that could null the subquery being pulled up.  This improves
the planner's ability to recognize constant join quals, and probably
helps with detection of common sort keys (equivalence classes) as well.