## What changes were proposed in this pull request?

`BroadcastHint` should use child's statistics and set `isBroadcastable` to true.

## How was this patch tested?

Added a new stats estimation test for `BroadcastHint`.

Author: wangzhenhua <wangzhenhua@huawei.com>

Closes #17504 from wzhfy/broadcastHintEstimation.