Optimize PredicateLockTuple().

PredicateLockTuple() has a fast exit if tuple was written by the current
transaction, as in that case it already has a lock.  This check can be
performed using TransactionIdIsCurrentTransactionId() instead of
SubTransGetTopmostTransaction(), to avoid any chance of having to hit the
disk.

Author: Ashwin Agrawal, based on a suggestion from Andres Freund
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CALfoeiv0k3hkEb3Oqk%3DziWqtyk2Jys1UOK5hwRBNeANT_yX%2Bng%40mail.gmail.com

src/backend/storage/lmgr/predicate.c

@@ -2549,7 +2549,6 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
 {
 	PREDICATELOCKTARGETTAG tag;
 	ItemPointer tid;
-	TransactionId targetxmin;
 
 	if (!SerializationNeededForRead(relation, snapshot))
 		return;
@@ -2559,24 +2558,9 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
 	 */
 	if (relation->rd_index == NULL)
 	{
-		TransactionId myxid;
-
-		targetxmin = HeapTupleHeaderGetXmin(tuple->t_data);
-
-		myxid = GetTopTransactionIdIfAny();
-		if (TransactionIdIsValid(myxid))
-		{
-			if (TransactionIdFollowsOrEquals(targetxmin, TransactionXmin))
-			{
-				TransactionId xid = SubTransGetTopmostTransaction(targetxmin);
-
-				if (TransactionIdEquals(xid, myxid))
-				{
-					/* We wrote it; we already have a write lock. */
-					return;
-				}
-			}
-		}
+		/* If we wrote it; we already have a write lock. */
+		if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetXmin(tuple->t_data)))
+			return;
 	}
 
 	/*