Re: A new strategy for pull-up correlated ANY_SUBLINK

Looking again at that contain_vars_of_level restriction, I think the
reason for it was just to avoid making a FROM subquery that has outer
references, and the reason we needed to avoid that was merely that we
didn't have LATERAL at the time.  So I experimented with the attached.
It seems to work, in that we don't get wrong answers from any of the
small number of places that are affected.  (I wonder though whether
those test cases still test what they were intended to, particularly
the postgres_fdw one.  We might have to try to hack them some more
to not get affected by this optimization.)  Could do with more test
cases, no doubt.

One thing I'm not at all clear about is whether we need to restrict
the optimization so that it doesn't occur if the subquery contains
outer references falling outside available_rels.  I think that that
case is covered by is_simple_subquery() deciding later to not pull up
the subquery based on LATERAL restrictions, but maybe that misses
something.