Assert that RTIs of joined rels are discoverable from join plans.

Assert that RTIs of joined rels are discoverable from join plans.

Every RTI that appears in the joinrel's relid set should be findable
via the outer or inner plan, except for join RTIs which aren't
necessarily preserved in the final plan. This is a requirement if
we want to be able to reliably determine the chosen join order from
the final plan, although it's not sufficient for that goal of itself,
due to further problems created by setrefs-time processing.

Note that this depends on the earlier commit to add a relids field to
Result nodes; without that change, a join tree involving two or more
Result nodes would be fundamentally ambiguous (and even a join tree
involving one could only be interpreted by guessing at its origin).