planner: explain explore outputs nothing

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
mysql> plan replayer load /Users/zhangyuanjia/Downloads/replayer1106.zip

mysql> source /Users/zhangyuanjia/Downloads/replayer1106/sql/explain_explaore.sql;
Empty set (2.509 sec)
```

Contact me to get the plan replayer file internally.

`Explain Explore` output nothing, which is unexpected.

The reason is that we'll construct a set of hint from all explored plans to reproduce them, but due to some limitation of the process of constructing these hints (implemented in `GenHintsFromFlatPlan`), these hints might not be able to reproduce its corresponding plan. Then in this case we just filter out this plan.
Since the query in this plan-replayer is relatively complex, we all its possible plans are not be able to reproduce via our constructed hints, then then output is empty.
We need to enhance `GenHintsFromFlatPlan` or figure out another way to reproduce the plan.

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

Some plans

### 3. What did you see instead (Required)

Empty

### 4. What is your TiDB version? (Required)

Master