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.