Don't reject ROW_MARK_REFERENCE rowmarks for materialized views.

We should allow this so that matviews can be referenced in UPDATE/DELETE
statements in READ COMMITTED isolation level.  The requirement for that
is that a re-fetch by TID will see the same row version the query saw
earlier, which is true of matviews, so there's no reason for the
restriction.  Per bug #9398.

Michael Paquier, after a suggestion by me