[PATCH] Check operator when creating unique index on partition table

Hi,

I found that things could go wrong in some cases, when the unique index and the partition key use different opclass.

For example:
```
CREATE TABLE ptop_test (a int, b int, c int) PARTITION BY LIST (a);
CREATE TABLE ptop_test_p1 PARTITION OF ptop_test FOR VALUES IN ('1');
CREATE TABLE ptop_test_m1 PARTITION OF ptop_test FOR VALUES IN ('-1');
CREATE UNIQUE INDEX ptop_test_unq_abs_a ON ptop_test (a abs_int_btree_ops); -- this should fail
```
In this example, `abs_int_btree_ops` is a opclass whose equality operator is customized to consider ‘-1’ and ‘1’ as equal.
So the unique index should not be allowed to create, since ‘-1’ and ‘1’ will be put in different partition.

The attached patch should fix this problem.