Statistics' async load loads more items than current sync load

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Before #56813, take the following SQL as an example:

table t(a int, b int, cint, index idx(a, b));
select * from t where a > 1;

Both sync load and async load will load a's full stats, idx's full stats, and the NDV/NullCount info of columns b and c.

After #56813, sync load will not load the stats of b and c anymore. But we forget to modify the async load.
So as a result, b and c's NDV/NullCount will be loaded finally.
It's a mistake and we should fix it.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

current master.