BUG #14341: insert on conflict (key_column) cann't used with partition table.

The following bug has been logged on the website:

Bug reference: 14341
Logged by: Zhou Digoal
Email address: digoal(at)126(dot)com
PostgreSQL version: 9.6rc1
Operating system: CentOS 6.x x64
Description:

```
postgres=# \d test
Table "postgres.test"
Column | Type | Modifiers
----------+-----------------------------+-----------
id | integer | not null
info | text |
crt_time | timestamp without time zone |
Indexes:
"test_pkey" PRIMARY KEY, btree (id)
Number of child tables: 100 (Use \d+ to list them.)

postgres=# insert into test(id,info,crt_time) values(1, md5(random()::text),
now()) on conflict (id) do update set
info=excluded.info,crt_time=excluded.crt_time;
ERROR: XX000: unexpected failure to find arbiter index
LOCATION: ExecCheckIndexConstraints, execIndexing.c:594
```