Fix inconsistent trigger OIDs between master and segments.

Running gpcheckcat after constraints installcheck test shows trigger metadata
inconsistencies between the master and segments. This issue was introduced
in the 8.3 merge. If CreateTrigger Statement has a trigger oid already, we
should reuse it like before the 8.3 merge.

src/backend/commands/trigger.c

@@ -241,7 +241,10 @@ CreateTrigger(CreateTrigStmt *stmt, Oid constraintOid)
 	 */
 	tgrel = heap_open(TriggerRelationId, RowExclusiveLock);
 
-	trigoid = GetNewOid(tgrel);
+	if (OidIsValid(stmt->trigOid))
+		trigoid = stmt->trigOid;
+	else
+		trigoid = GetNewOid(tgrel);
 
 	/*
 	 * If trigger is for an RI constraint, the passed-in name is the