BUG #14132: ON CONFLICT not inferring unique index with bigints

The following bug has been logged on the website:

Bug reference:      14132
Logged by:          Reynold Smith
Email address:      fiercetuba@protonmail.com
PostgreSQL version: 9.5.2
Operating system:   Linux
Description:

This fails, but change bigint to int and it works fine.

```
CREATE TABLE testtable(
  a  serial PRIMARY KEY,
  b  bigint,
  c  bigint
);

CREATE UNIQUE INDEX super_unique_idx ON testtable( COALESCE(b, 0),
COALESCE(c, 0));

INSERT INTO testtable(b, c)
VALUES (1, 2)
ON CONFLICT ((COALESCE(b, 0)), (COALESCE(c, 0))) DO NOTHING;
```