Further tighten nullingrel marking rules in build_joinrel_tlist().

Further tighten nullingrel marking rules in build_joinrel_tlist().

The code I added in fee7b77b9 could misbehave if commute_above_r
contains multiple relids.  While adding too many relids here is
probably harmless (pre-fee7b77b9, we did it all the time), it's
not very expensive to be accurate: we just have to intersect
commute_above_r with the join's relids.

Discussion: https://postgr.es/m/17781-c0405c8b3cd5e072@postgresql.org