Fix slot type assumptions for nodeGather[Merge].

Fix slot type assumptions for nodeGather[Merge].

The assumption made in 1a0586de3657c was wrong, as evidenced by
buildfarm failure on locust, which runs with
force_parallel_mode=regress.  The tuples accessed in either nodes are
in the outer slot, and we can't trivially rely on the slot type being
known because the leader might execute the subsidiary node directly,
or via the tuple queue on a worker. In the latter case the tuple will
always be a heaptuple slot, but in the former, it'll be whatever the
subsidiary node returns.