The explain result for some PointGet plans should be improved

## Enhancement
When some PointGet plans will happen twice in a query, but the explain result can not see that. It would be misleading to investigate the problem.

```
mysql> create table t(a double, b int, primary key(a) nonclustered);
Query OK, 0 rows affected (0.02 sec)

mysql> explain select * from t where a = 1;
+-------------+---------+------+---------------------------+---------------+
| id | estRows | task | access object | operator info |
+-------------+---------+------+---------------------------+---------------+
| Point_Get_1 | 1.00 | root | table:t, index:PRIMARY(a) | |
+-------------+---------+------+---------------------------+---------------+
1 row in set (0.00 sec)
```

For the non-clustered index, it will use the primary key to get the row_id first and use the row_id to get the row values. To sum up, it will use PointGet twice to get the result. But the explain can only show once.