Make SP-GiST redirect cleanup more aggressive.

Commit 61b313e made VACUUM pass down a heaprel to index AM bulkdelete
and vacuumcleanup routines.  Although this was primarily intended as
preparation for logical decoding on standbys, it also made it easy to
correct an old deficiency in how we determine how to cleanup SP-GiST
redirect and placeholder tuples.

Pass the heaprel to GlobalVisTestFor() during cleanup of redirect and
placeholder tuples, rather than pessimistically passing NULL.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/02392033-f030-a3c8-c7d0-5c27eb529fec@gmail.com

Author: petergeoghegan

src/backend/access/spgist/spgvacuum.c

@@ -507,8 +507,7 @@ vacuumRedirectAndPlaceholder(Relation index, Relation heaprel, Buffer buffer)
 	xlrec.nToPlaceholder = 0;
 	xlrec.snapshotConflictHorizon = InvalidTransactionId;
 
-	/* XXX: providing heap relation would allow more pruning */
-	vistest = GlobalVisTestFor(NULL);
+	vistest = GlobalVisTestFor(heaprel);
 
 	START_CRIT_SECTION();