Optimize TransactionIdIsCurrentTransactionId().

If the passed in xid is the current top transaction, we can do a fast
check and exit early.  This should work well for the current heap but
also works very well for proposed AMs that don't use a separate xid
for subtransactions.

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/access/transam/xact.c

@@ -871,6 +871,9 @@ TransactionIdIsCurrentTransactionId(TransactionId xid)
 	if (!TransactionIdIsNormal(xid))
 		return false;
 
+	if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
+		return true;
+
 	/*
 	 * In parallel workers, the XIDs we must consider as current are stored in
 	 * ParallelCurrentXids rather than the transaction-state stack.  Note that