Multi-schema adding index should not trigger merge step

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Add multiple indexes for 1TB sysbench table, for example

alter table sbtest1 
  add index idx_p(pad),
  add index idx_k(k),
  add index idx_c(c);

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

TiDB should not run merge sort because the dataset is not large enough.

3. What did you see instead (Required)

TiDB run merge sort step.

It may take more than 100% time to complete.

4. What is your TiDB version? (Required)

69c26ae