Trying to pull up EXPR SubLinks

Currently we will not consider EXPR_SUBLINK when pulling up sublinks and
this would cause performance issues for some queries with the form of:
'a > (SELECT agg(b) from ...)' as described in [1].

So here is a patch as an attempt to pull up EXPR SubLinks. The idea,
which is based on Greenplum's implementation, is to perform the
following transformation.

For query:

select * from foo where foo.a >
    (select avg(bar.a) from bar where foo.b = bar.b);

we transform it to:

select * from foo inner join
    (select bar.b, avg(bar.a) as avg from bar group by bar.b) sub
on foo.b = sub.b and foo.a > sub.avg;

To do that, we recurse through the quals in sub-select and extract quals
of form 'foo(outervar) = bar(innervar)' and then according to innervars
we make new SortGroupClause items and TargetEntry items for sub-select.
And at last we pull up the sub-select into upper range table.

As a result, the plan would change as:

FROM

               QUERY PLAN
----------------------------------------
 Seq Scan on foo
   Filter: ((a)::numeric > (SubPlan 1))
   SubPlan 1
     ->  Aggregate
           ->  Seq Scan on bar
                 Filter: (foo.b = b)
(6 rows)

TO

                    QUERY PLAN
--------------------------------------------------
 Hash Join
   Hash Cond: (foo.b = bar.b)
   Join Filter: ((foo.a)::numeric > (avg(bar.a)))
   ->  Seq Scan on foo
   ->  Hash
         ->  HashAggregate
               Group Key: bar.b
               ->  Seq Scan on bar
(8 rows)

The patch works but still in draft stage. Post it here to see if it is
the right thing we want.

[1] https://www.postgresql.org/message-id/flat/CAKU4AWodctmbU%2BZj6U83y_RniQk0UeXBvKH1ZaJ%3DLR_iC90GOw%40mail.gmail.com

Thanks

Richard