Further tweaking of logic that decides when to materialize an uncorrelated

Further tweaking of logic that decides when to materialize an uncorrelated
subplan: do it if subplan has subplans itself, and always do it if the
subplan is an indexscan.  (I originally set it to materialize an indexscan
only if the indexqual is fairly selective, but I dunno what I was
thinking ... an unselective indexscan is still expensive ...)