Self review of previous patch. Fix assumption that xmax >= xmin.

src/backend/access/heap/heapam.c

@@ -3809,12 +3809,10 @@ HeapTupleHeaderAdvanceLatestRemovedXid(HeapTupleHeader tuple,
 		 !(tuple->t_infomask & HEAP_XMIN_INVALID) &&
 		 TransactionIdDidCommit(xmin)))
 	{
-		if (TransactionIdFollows(xmax, xmin))
-		{
-			if (TransactionIdFollows(xmax, *latestRemovedXid))
+		if (xmax != xmin &&
+			TransactionIdFollows(xmax, *latestRemovedXid))
 				*latestRemovedXid = xmax;
 	}
-	}
 
 	/* *latestRemovedXid may still be invalid at end */
 }