Fix systable_recheck_tuple() for MVCC scan snapshots.
Since this function assumed non-MVCC snapshots, it broke when commit
568d4138c646cd7cd8a837ac244ef2caf27c6bb8 switched its one caller from
SnapshotNow scans to MVCC-snapshot scans.
Reviewed by Robert Haas, Tom Lane and Andres Freund.