Better handle pseudotypes as partition keys

We fail to handle polymorphic types properly when they are used as
partition keys: we were unnecessarily adding a RelabelType node on top,
which confuses code examining the nodes.  In particular, this makes
predtest.c-based partition pruning not to work, and ruleutils.c to emit
expressions that are uglier than needed.  Fix it by not adding RelabelType
when not needed.

In master/11 the new pruning code is separate so it doesn't suffer from
this problem, since we already fixed it (in essentially the same way) in
e5dcbb88a1, which also added a few tests; back-patch those tests to
pg10 also.  But since UPDATE/DELETE still uses predtest.c in pg11, this
change improves partitioning for those cases too.  Add tests for this.
The ruleutils.c behavior change is relevant in pg11/master too.

Co-authored-by: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/54745d13-7ed4-54ac-97d8-ea1eec95ae25@lab.ntt.co.jp

src/test/regress/expected/create_table.out

@@ -882,7 +882,7 @@ CREATE TABLE arrlp12 PARTITION OF arrlp FOR VALUES IN ('{1}', '{2}');
 --------+-----------+-----------+----------+---------+----------+--------------+-------------
  a      | integer[] |           |          |         | extended |              | 
 Partition of: arrlp FOR VALUES IN ('{1}', '{2}')
-Partition constraint: ((a IS NOT NULL) AND (((a)::anyarray OPERATOR(pg_catalog.=) '{1}'::integer[]) OR ((a)::anyarray OPERATOR(pg_catalog.=) '{2}'::integer[])))
+Partition constraint: ((a IS NOT NULL) AND ((a = '{1}'::integer[]) OR (a = '{2}'::integer[])))
 
 DROP TABLE arrlp;
 -- partition on boolean column

src/test/regress/expected/partition_prune.out

@@ -2817,6 +2817,24 @@ explain (costs off) select * from pp_arrpart where a in ('{4, 5}', '{1}');
          Filter: ((a = '{4,5}'::integer[]) OR (a = '{1}'::integer[]))
 (5 rows)
 
+explain (costs off) update pp_arrpart set a = a where a = '{1}';
+               QUERY PLAN               
+----------------------------------------
+ Update on pp_arrpart
+   Update on pp_arrpart1
+   ->  Seq Scan on pp_arrpart1
+         Filter: (a = '{1}'::integer[])
+(4 rows)
+
+explain (costs off) delete from pp_arrpart where a = '{1}';
+               QUERY PLAN               
+----------------------------------------
+ Delete on pp_arrpart
+   Delete on pp_arrpart1
+   ->  Seq Scan on pp_arrpart1
+         Filter: (a = '{1}'::integer[])
+(4 rows)
+
 drop table pp_arrpart;
 -- array type hash partition key
 create table pph_arrpart (a int[]) partition by hash (a);

src/test/regress/sql/partition_prune.sql

@@ -721,6 +721,8 @@ create table pp_arrpart2 partition of pp_arrpart for values in ('{2, 3}', '{4, 5
 explain (costs off) select * from pp_arrpart where a = '{1}';
 explain (costs off) select * from pp_arrpart where a = '{1, 2}';
 explain (costs off) select * from pp_arrpart where a in ('{4, 5}', '{1}');
+explain (costs off) update pp_arrpart set a = a where a = '{1}';
+explain (costs off) delete from pp_arrpart where a = '{1}';
 drop table pp_arrpart;
 
 -- array type hash partition key