Generate EquivalenceClass members for partitionwise child join rels.

Commit d25ea012 got rid of what I thought were entirely unnecessary
derived child expressions in EquivalenceClasses for EC members that
mention multiple baserels.  But it turns out that some of the child
expressions that code created are necessary for partitionwise joins,
else we fail to find matching pathkeys for Sort nodes.  (This happens
only for certain shapes of the resulting plan; it may be that
partitionwise aggregation is also necessary to show the failure,
though I'm not sure of that.)

Reverting that commit entirely would be quite painful performance-wise
for large partition sets.  So instead, add code that explicitly
generates child expressions that match only partitionwise child join
rels we have actually generated.

Per report from Justin Pryzby.  (Amit Langote noticed the problem
earlier, though it's not clear if he recognized then that it could
result in a planner error, not merely failure to exploit partitionwise
join, in the code as-committed.)  Back-patch to v12 where commit
d25ea012 came in.

Amit Langote, with lots of kibitzing from me

Discussion: https://postgr.es/m/CA+HiwqG2WVUGmLJqtR0tPFhniO=H=9qQ+Z3L_ZC+Y3-EVQHFGg@mail.gmail.com
Discussion: https://postgr.es/m/20191011143703.GN10470@telsasoft.com