Enable GPORCA to generate better plans for non-correlated exists subquery in the WHERE clause

Consider the following exists subquery, `(select * from bar)`. GPORCA generates an elaborate count based implementation of this subquery. If bar is a fact table, the count is going to be expensive.

```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
                                                    QUERY PLAN
------------------------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice3; segments: 3)  (cost=0.00..1368262.79 rows=400324 width=8)
   ->  Nested Loop  (cost=0.00..1368250.86 rows=133442 width=8)
         Join Filter: true
         ->  Table Scan on foo  (cost=0.00..461.91 rows=133442 width=8)
               Filter: a = b
         ->  Materialize  (cost=0.00..438.57 rows=1 width=1)
               ->  Broadcast Motion 1:3  (slice2)  (cost=0.00..438.57 rows=3 width=1)
                     ->  Result  (cost=0.00..438.57 rows=1 width=1)
                           Filter: (count((count()))) > 0::bigint
                           ->  Aggregate  (cost=0.00..438.57 rows=1 width=8)
                                 ->  Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..438.57 rows=1 width=8)
                                       ->  Aggregate  (cost=0.00..438.57 rows=1 width=8)
                                             ->  Table Scan on bar  (cost=0.00..437.95 rows=332395 width=1)
 Optimizer status: PQO version 2.35.1
(14 rows)
```
Planner on the other hand uses LIMIT as shown in the INIT plan.

```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
                                           QUERY PLAN
------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice2; segments: 3)  (cost=0.03..13611.14 rows=1001 width=8)
   ->  Result  (cost=0.03..13611.14 rows=334 width=8)
         One-Time Filter: $0
         InitPlan  (slice3)
           ->  Limit  (cost=0.00..0.03 rows=1 width=0)
                 ->  Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..0.03 rows=1 width=0)
                       ->  Limit  (cost=0.00..0.01 rows=1 width=0)
                             ->  Seq Scan on bar  (cost=0.00..11072.84 rows=332395 width=0)
         ->  Seq Scan on foo  (cost=0.00..13611.11 rows=334 width=8)
               Filter: a = b
 Settings:  optimizer=off
 Optimizer status: legacy query optimizer
(12 rows)
```

While GPORCA doesnot support init-plan, we can nevertheless generate a better plan by using LIMIT instead of count. After this PR, GPORCA will generate the following plan with LIMIT clause.

```
vraghavan=# explain select * from foo where foo.a = foo.b and exists (select * from bar);
                                                 QUERY PLAN
------------------------------------------------------------------------------------------------------------
 Gather Motion 3:1  (slice3; segments: 3)  (cost=0.00..1368262.73 rows=400324 width=8)
   ->  Nested Loop EXISTS Join  (cost=0.00..1368250.80 rows=133442 width=8)
         Join Filter: true
         ->  Table Scan on foo  (cost=0.00..461.91 rows=133442 width=8)
               Filter: a = b
         ->  Materialize  (cost=0.00..438.57 rows=1 width=1)
               ->  Broadcast Motion 1:3  (slice2)  (cost=0.00..438.57 rows=3 width=1)
                     ->  Limit  (cost=0.00..438.57 rows=1 width=1)
                           ->  Gather Motion 3:1  (slice1; segments: 3)  (cost=0.00..438.57 rows=1 width=1)
                                 ->  Limit  (cost=0.00..438.57 rows=1 width=1)
                                       ->  Table Scan on bar  (cost=0.00..437.95 rows=332395 width=1)
 Optimizer status: PQO version 2.35.1
(12 rows)
```