point query on unique index will make estimation row count inconsistent

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

create a table, inserting some rows. Make sure that there's modify_count after analyze.

Run a query that contains PointGet in normal execution.
like
```
CREATE TABLE `t` (
`a` varchar(20) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
PRIMARY KEY (`a`) /*T![clustered_index] CLUSTERED */
);

explain select /*+ HASH_JOIN(t1, t2) */ * from t t1, t t2 where t1.c=t2.c and t1.a='1' and t1.b > 1;
```

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

```
mysql> explain select /*+ HASH_JOIN(t1, t2) */ * from t t1, t t2 where t1.c=t2.c and t1.a='1' and t1.b > 1;
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
| HashJoin_9 | 1.00 | root | | inner join, equal:[eq(test.t.c, test.t.c)] |
| ├─Selection_11(Build) | 1.00 | root | | gt(test.t.b, 1), not(isnull(test.t.c)) |
| │ └─Point_Get_10 | 1.00 | root | table:t, clustered index:PRIMARY(a) | |
| └─TableReader_14(Probe) | 1536.00 | root | | data:Selection_13 |
| └─Selection_13 | 1536.00 | cop[tikv] | | not(isnull(test.t.c)) |
| └─TableFullScan_12 | 1536.00 | cop[tikv] | table:t2 | keep order:false |
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
6 rows in set (0.00 sec)
```

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

```
mysql> explain select /*+ HASH_JOIN(t1, t2) */ * from t t1, t t2 where t1.c=t2.c and t1.a='1' and t1.b > 1;
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
| HashJoin_9 | 1.50 | root | | inner join, equal:[eq(test.t.c, test.t.c)] |
| ├─Selection_11(Build) | 1.50 | root | | gt(test.t.b, 1), not(isnull(test.t.c)) |
| │ └─Point_Get_10 | 1.00 | root | table:t, clustered index:PRIMARY(a) | |
| └─TableReader_14(Probe) | 1536.00 | root | | data:Selection_13 |
| └─Selection_13 | 1536.00 | cop[tikv] | | not(isnull(test.t.c)) |
| └─TableFullScan_12 | 1536.00 | cop[tikv] | table:t2 | keep order:false |
+-----------------------------+---------+-----------+-------------------------------------+--------------------------------------------+
6 rows in set (0.00 sec)
```
Selection_11 got an estimation larger than PointGet_10

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

master still has this problem.