With PostgreSQL, if you do "SELECT * FROM foo WHERE id = 1 and id = 2",
you get a Scan node on foo, with a gating Result node with False one-time
filter. That's what you also got on GPDB before this patch. Before merging
the equivalence classes patch, however, you got a Result node with
False one-time filter, with no subplan. This patch changes the behaviour
back to that.

Avoiding the planning saves some time during planning, as we don't need
to decide how to scan the relation. But more importantly, a stand-alone
Result node with no subplan can be executed on any segment, so the planner
can avoid unnecessary Motion nodes in some cases.