Ignore CatalogSnapshot when checking COPY FREEZE prerequisites.

This restores the ability, essentially lost in commit
ffaa44cb, to use COPY FREEZE under
REPEATABLE READ isolation.  Back-patch to 9.4, like that commit.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/CA+TgmoahWDm-7fperBxzU9uZ99LPMUmEpSXLTw9TmrOgzwnORw@mail.gmail.com