Prevent generation of invalid plans for RIGHT or FULL joins with multiple

join clauses.  The mergejoin executor wants all the join clauses to appear
as merge quals, not as extra joinquals, for these kinds of joins.  But the
planner would consider plans in which partially-sorted input paths were
used, leading to only some of the join clauses becoming merge quals.
This is fine for inner/left joins, not fine for right/full joins.