For FROM-clause subqueries that cannot be flattened, try to push relevant

For FROM-clause subqueries that cannot be flattened, try to push relevant
WHERE clause terms of the outer query down into the subquery in order to help
the subquery run faster and/or use less memory.
Cherry-pick from [6df18e949d36].  Still need to backport bug fixes associated
with that check-in.

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