USE INDEX(PRIMARY) will remove tiflash path

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

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

Tiflash path should be reserved since it is a primary key table path.

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

current master.

[qw4990]

Emm, it seems this issue depends on how we define 'Primary'. Now 'Primary' only refer to 'Primary Key on TiKV'.