pgsql: Repair oversight in 8.2 change that improved the handling of

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execAmi.c (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execAmi.c.diff?r1=1.90&r2=1.91)
        nodeResult.c (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeResult.c.diff?r1=1.37&r2=1.38)
    pgsql/src/include/executor:
        nodeResult.h (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/nodeResult.h.diff?r1=1.23&r2=1.24)