orderby doesn't use the index when to init stats

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SQL in the init stats like load bucket/topn have orderby. but when to explain the SQL, all of them do not use index to orderby

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

use index

3. What did you see instead (Required)

not use index

4. What is your TiDB version? (Required)