If a PlaceHolderVar is to be evaluated at a join relation, but
its value is only needed there and not at higher levels, we neglected
to update the joinrel's direct_lateral_relids to include the PHV's
source rel. This causes problems because join_is_legal() then won't
allow joining the joinrel to the PHV's source rel at all, leading
to "failed to build any N-way joins" planner failures.
Per report from Andreas Seltenreich. Back-patch to 9.5
where the problem originated.
Discussion: https://postgr.es/m/87blfgqa4t.fsf@aurora.ydns.eu
caecab229a