Fix planner to do the right thing when a degenerate outer join (one whose

Fix planner to do the right thing when a degenerate outer join (one whose
joinclause doesn't use any outer-side vars) requires a "bushy" plan to be
created.  The normal heuristic to avoid joins with no joinclause has to be
overridden in that case.  Problem is new in 8.2; before that we forced the
outer join order anyway.  Per example from Teodor.