Return early from distributed snapshot check if local xid is not normal.

Only normal xids can have distributed xids, so no point trying to check
distributed log and lookout for its corresponding distributed xid. Not sure how
much performance this impacts but its more important to not go chasing down
non-existent file, corresponding to frozen xids. Though currently for
distributed log it treats that as local transaction but just lot of extra work
to get that answer.

src/backend/cdb/cdbdistributedsnapshot.c

@@ -37,6 +37,13 @@ DistributedSnapshotWithLocalMapping_CommittedTest(
 	bool							found;
 	DistributedTransactionId		distribXid = InvalidDistributedTransactionId;
 
+	/*
+	 * Return early if local xid is not normal as it cannot have distributed
+	 * xid associated with it.
+	 */
+	if (!TransactionIdIsNormal(localXid))
+		return DISTRIBUTEDSNAPSHOT_COMMITTED_IGNORE;
+
 	count = header->count;
 
 	/*