Remove incidental dependencies on partitioned_rels lists.

Remove incidental dependencies on partitioned_rels lists.

It turns out that the calculation of [Merge]AppendPath.partitioned_rels
in allpaths.c is faulty and sometimes omits relevant non-leaf partitions,
allowing an assertion added by commit a929e17e5a8 to trigger.  Rather
than fix that, it seems better to get rid of those fields altogether.
We don't really need the info until create_plan time, and calculating
it once for the selected plan should be cheaper than calculating it
for each append path we consider.

This patch undoes a couple of very minor uses of the partitioned_rels
values.

createplan.c was testing for nil-ness to optimize away the preparatory
work for make_partition_pruneinfo().  That is worth doing if the check
is nigh free, but it's not worth going to any great lengths to avoid.

create_append_path() was testing for nil-ness as part of deciding how
to set up ParamPathInfo for an AppendPath.  I replaced that with a
check for the appendrel's parent rel being partitioned.  That's not
quite the same thing but should cover most cases.  If we note any
interesting loss of optimizations, we can dumb this down to just
always use the more expensive method when the parent is a baserel.

Discussion: https://postgr.es/m/87sg8tqhsl.fsf@aurora.ydns.eu
Discussion: https://postgr.es/m/CAJKUy5gCXDSmFs2c=R+VGgn7FiYcLCsEFEuDNNLGfoha=pBE_g@mail.gmail.com