Improve handling of dead tuples in hash indexes.

When squeezing a bucket during vacuum, it's not necessary to retain
any tuples already marked as dead, so ignore them when deciding which
tuples must be moved in order to empty a bucket page.  Similarly, when
splitting a bucket, relocating dead tuples to the new bucket is a
waste of effort; instead, just ignore them.

Amit Kapila, reviewed by me.  Testing help provided by Ashutosh
Sharma.

src/backend/access/hash/hashovfl.c

@@ -656,6 +656,10 @@ _hash_squeezebucket(Relation rel,
 			IndexTuple	itup;
 			Size		itemsz;
 
+			/* skip dead tuples */
+			if (ItemIdIsDead(PageGetItemId(rpage, roffnum)))
+				continue;
+
 			itup = (IndexTuple) PageGetItem(rpage,
 											PageGetItemId(rpage, roffnum));
 			itemsz = IndexTupleDSize(*itup);

src/backend/access/hash/hashpage.c

@@ -811,6 +811,10 @@ _hash_splitbucket(Relation rel,
 			Size		itemsz;
 			Bucket		bucket;
 
+			/* skip dead tuples */
+			if (ItemIdIsDead(PageGetItemId(opage, ooffnum)))
+				continue;
+
 			/*
 			 * Fetch the item's hash key (conveniently stored in the item) and
 			 * determine which bucket it now belongs in.