pgsql: Fix pull_up_sublinks' failure to handle nested pull-up opportuni

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/dcc685debb02c507b88aa8052f9e6334a7c3f830

Modified Files
--------------
src/backend/optimizer/plan/subselect.c | 5 +++
src/backend/optimizer/prep/prepjointree.c | 41 ++++++++++++++++++++++++++--
2 files changed, 43 insertions(+), 3 deletions(-)