Fix planning of parameterized appendrel paths with expensive join quals.

The code in set_append_rel_pathlist() for building parameterized paths
for append relations (inheritance and UNION ALL combinations) supposed
that the cheapest regular path for a child relation would still be cheapest
when reparameterized.  Which might not be the case, particularly if the
added join conditions are expensive to compute, as in a recent example from
Jeff Janes.  Fix it to compare child path costs *after* reparameterizing.
We can short-circuit that if the cheapest pre-existing path is already
parameterized correctly, which seems likely to be true often enough to be
worth checking for.

Back-patch to 9.2 where parameterized paths were introduced.