Fix pull_up_sublinks' failure to handle nested pull-up opportunities.

After finding an EXISTS or ANY sub-select that can be converted to a
semi-join or anti-join, we should recurse into the body of the sub-select.
This allows cases such as EXISTS-within-EXISTS to be optimized properly.
The original coding would leave the lower sub-select as a SubLink, which
is no better and often worse than what we can do with a join.  Per example
from Wayne Conrad.

Back-patch to 8.4.  There is a related issue in older versions' handling
of pull_up_IN_clauses, but they're lame enough anyway about the whole area
that it seems not worth the extra work to try to fix.

src/backend/optimizer/plan/subselect.c

@@ -1065,6 +1065,11 @@ SS_process_ctes(PlannerInfo *root)
  * (Notionally, we replace the SubLink with a constant TRUE, then elide the
  * redundant constant from the qual.)
  *
+ * On success, the caller is also responsible for recursively applying
+ * pull_up_sublinks processing to the rarg and quals of the returned JoinExpr.
+ * (On failure, there is no need to do anything, since pull_up_sublinks will
+ * be applied when we recursively plan the sub-select.)
+ *
  * Side effects of a successful conversion include adding the SubLink's
  * subselect to the query's rangetable, so that it can be referenced in
  * the JoinExpr's rarg.