tidb: Can not swap values by a multi-tables update

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t1, t2;
create table t1 (c_str varchar(40));
create table t2 (c_str varchar(40));
insert into t1 values ('Alice');
insert into t2 values ('Bob');
select t1.c_str, t2.c_str from t1, t2 where t1.c_str <= t2.c_str;
update t1, t2 set t1.c_str = t2.c_str, t2.c_str = t1.c_str where t1.c_str <= t2.c_str;
select t1.c_str, t2.c_str from t1, t2 where t1.c_str <= t2.c_str;

2. What did you expect to see? (Required)

The values of t1.c_str and t2.c_str should be swapped in the above case, that is, the last selection should return an empty set.

3. What did you see instead (Required)

mysql> select t1.c_str, t2.c_str from t1, t2 where t1.c_str <= t2.c_str;
+-------+-------+
| c_str | c_str |
+-------+-------+
| Alice | Bob   |
+-------+-------+
1 row in set (0.06 sec)

mysql> update t1, t2 set t1.c_str = t2.c_str, t2.c_str = t1.c_str where t1.c_str <= t2.c_str;
Query OK, 1 row affected (0.06 sec)
Rows matched: 2  Changed: 1  Warnings: 0

mysql> select t1.c_str, t2.c_str from t1, t2 where t1.c_str <= t2.c_str;
+-------+-------+
| c_str | c_str |
+-------+-------+
| Bob   | Bob   |
+-------+-------+
1 row in set (0.05 sec)

4. What is your TiDB version? (Required)

master @ 8978773f5e3d43a100550e6babea9904a99e5938 release-4.0 @ 3948c7ba7b5d4a081abf2162b791ab67fe650882