Wrong behavior when primary key is Unsigned int.

[winoros]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t1 (a bigint unsigned not null, primary key(a));
insert into t1 values (18446744073709551615);
insert into t1 values (1);
select * from t1 order by a;

2. What did you expect to see?

Two row, first is 1, second is 18446744073709551615.

3. What did you see instead?

Two row, but first is 18446744073709551615, second is 1.

4. What version of TiDB are you using (tidb-server -V)?