quickly get total count from index/column

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Recently, I found that the map iter time here has become significantly longer.

Compared with the older code, you will find that the current code will traverse the column statistics after traversing the index, which makes the time here longer.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)