Mutation checker can use up to 30% of CPU time when executing insert into select

[ekexium]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

insert into t_1 select * from t_2 where t_2 is a huge table.

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

The mutation checker doesn't have a large impact on performance.

3. What did you see instead (Required)

The mutation checker uses 30% of the CPU time.

4. What is your TiDB version? (Required)