Fix some more problems with nested append relations.

 Fix some more problems with nested append relations.
 
 As of commit a87c72915 (which later got backpatched as far as 9.1),
 we're explicitly supporting the notion that append relations can be
 nested; this can occur when UNION ALL constructs are nested, or when
 a UNION ALL contains a table with inheritance children.
 
 Bug #11457 from Nelson Page, as well as an earlier report from Elvis
 Pranskevichus, showed that there were still nasty bugs associated with such
 cases: in particular the EquivalenceClass mechanism could try to generate
 "join" clauses connecting an appendrel child to some grandparent appendrel,
 which would result in assertion failures or bogus plans.
 
 Upon investigation I concluded that all current callers of
 find_childrel_appendrelinfo() need to be fixed to explicitly consider
 multiple levels of parent appendrels.  The most complex fix was in
 processing of "broken" EquivalenceClasses, which are ECs for which we have
 been unable to generate all the derived equality clauses we would like to
 because of missing cross-type equality operators in the underlying btree
 operator family.  That code path is more or less entirely untested by
 the regression tests to date, because no standard opfamilies have such
 holes in them.  So I wrote a new regression test script to try to exercise
 it a bit, which turned out to be quite a worthwhile activity as it exposed
 existing bugs in all supported branches.
 
 The present patch is essentially the same as far back as 9.2, which is
 where parameterized paths were introduced.  In 9.0 and 9.1, we only need
 to back-patch a small fragment of commit 5b7b5518d, which fixes failure to
 propagate out the original WHERE clauses when a broken EC contains constant
 members.  (The regression test case results show that these older branches
 are noticeably stupider than 9.2+ in terms of the quality of the plans
 generated; but we don't really care about plan quality in such cases,
 only that the plan not be outright wrong.  A more invasive fix in the
 older branches would not be a good idea anyway from a plan-stability
 standpoint.)