`REGEXP_INSTR` is not compatible with MySQL for some cases

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

`SELECT REGEXP_INSTR('a\nb', 'a.b', 1, 1, 0, 'n');`

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

```
mysql> SELECT REGEXP_INSTR('a\nb', 'a.b', 1, 1, 0, 'n');
+-------------------------------------------+
| REGEXP_INSTR('a\nb', 'a.b', 1, 1, 0, 'n') |
+-------------------------------------------+
| 1 |
+-------------------------------------------+
1 row in set (0.00 sec)
```

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

```
mysql> SELECT REGEXP_INSTR('a\nb', 'a.b', 1, 1, 0, 'n');
ERROR 1139 (42000): Got error 'Invalid match type' from regexp
```

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