Avoid considering both sort directions as equally useful for merging.

This doubles the planning workload for mergejoins while not actually
accomplishing much.  The only useful case is where one of the directions
matches the query's ORDER BY request; therefore, put a thumb on the scales
in that direction, and otherwise arbitrarily consider only the ASC direction.
(This is a lot easier now than it would've been before 8.3, since we have
more semantic knowledge embedded in PathKeys now.)