caos.backends.pgsql: Push down paths with subqueries in EXISTS predicate
caos.backends.pgsql: Push down paths with subqueries in EXISTS predicate

Current coding in SQL query transformer expects paths with subqueries to
be processed _after_ regular paths so that subqueries link properly to
the main relation.  This prioritization failed to apply on EXISTS (subquery)
expressions, but it should.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb