no partition pruning when partitioning using array type

Hi.

I noticed that if you partition using a array type column, partition
pruning using constraint exclusion fails to work due to a minor problem.

Example:

create table p (a int[]) partition by list (a);
create table p1 partition of p for values in ('{1}');
create table p1 partition of p for values in ('{2, 3}', '{4, 5}');

explain select a from p where a = '{1}';
                        QUERY PLAN
|---------------------------------------------------------
 Append  (cost=0.00..54.00 rows=14 width=32)
   ->  Seq Scan on p1  (cost=0.00..27.00 rows=7 width=32)
         Filter: (a = '{1}'::integer[])
   ->  Seq Scan on p2  (cost=0.00..27.00 rows=7 width=32)
         Filter: (a = '{1}'::integer[])

explain select a from p where a = '{2, 3}';
                        QUERY PLAN
|---------------------------------------------------------
 Append  (cost=0.00..54.00 rows=14 width=32)
   ->  Seq Scan on p1  (cost=0.00..27.00 rows=7 width=32)
         Filter: (a = '{2,3}'::integer[])
   ->  Seq Scan on p2  (cost=0.00..27.00 rows=7 width=32)
         Filter: (a = '{2,3}'::integer[])
(5 rows)


In the case of array type partition key, make_partition_op_expr() will
have to put a RelabelType node on top of the partition key Var, after
having selected an = operator from the array_ops family.  The RelabelType
causes operator_predicate_proof() to fail to consider predicate leftop and
clause leftop as equal, because only one of them ends up having the
RelabelType attached to it.

As a simple measure, the attached patch teaches operator_predicate_proof()
to strip RelabelType nodes from all the nodes it compares using equal().
I also added a relevant test in partition_prune.sql.

Thoughts?

Thanks,
Amit