Repair oversight in 8.2 change that improved the handling of "pseudoconstant"

Repair oversight in 8.2 change that improved the handling of "pseudoconstant"
WHERE clauses.  createplan.c is now willing to stick a gating Result node
almost anywhere in the plan tree, and in particular one can wind up directly
underneath a MergeJoin node.  This means it had better be willing to handle
Mark/Restore.  Fortunately, that's trivial in such cases, since we can just
pass off the call to the input node (which the planner has previously ensured
can handle Mark/Restore).  Per report from Phil Frost.