Unify sublink state in auxiliary Query

This makes the following query at least not throw an unplanned subquery
error:

with f as (select 1 as u) INSERT INTO test(key, val) VALUES(3, 'fizz')
ON CONFLICT update set val = (select u from f)::text || '   s';

(previously, this also accidentally worked when VALUES() happened to
have a reference to the CTE f, too).