Type conversion issue for RAND()- little different

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
DROP TABLE t1;
CREATE TABLE t1(
day_of_week enum('0','1','2','3','4','5','6'),
bit1 bit(1),
bit3 bit(3)
);

INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8),
(1+RAND()*7, RAND()*2, RAND()*8);
INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8);
INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8);
INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
```

### 2. What did you expect to see? (Required)
```sql
mysql> CREATE TABLE t1(
-> day_of_week enum('0','1','2','3','4','5','6'),
-> bit1 bit(1),
-> bit3 bit(3)
-> );
Query OK, 0 rows affected (0.02 sec)

mysql>
mysql> INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8),
-> (1+RAND()*7, RAND()*2, RAND()*8);
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
Query OK, 8 rows affected (0.00 sec)
Records: 8 Duplicates: 0 Warnings: 0

```
### 3. What did you see instead (Required)
```sql
mysql> CREATE TABLE t1(
-> day_of_week enum('0','1','2','3','4','5','6'),
-> bit1 bit(1),
-> bit3 bit(3)
-> );
Query OK, 0 rows affected (0.13 sec)

mysql> INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8),
-> (1+RAND()*7, RAND()*2, RAND()*8);
ERROR 1406 (22001): Data too long for column 'bit1' at row 1
mysql> INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8);
ERROR 1406 (22001): Data too long for column 'bit1' at row 1
mysql> INSERT INTO t1 VALUES (1+RAND()*7, RAND()*2, RAND()*8);
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> INSERT INTO t1 SELECT 1+RAND()*7, RAND()*2, RAND()*8 FROM t1;
ERROR 1406 (22001): Data Too Long, field len 1

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

```sql
| Release Version: v6.0.0
Edition: Community
Git Commit Hash: 36a9810441ca0e496cbd22064af274b3be771081
Git Branch: heads/refs/tags/v6.0.0
UTC Build Time: 2022-03-31 10:27:25
GoVersion: go1.18
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

```