Unnecessary warning in unreachable case branch

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```sql
DROP TABLE IF EXISTS t0;
CREATE TABLE t0(c0 INT);
INSERT INTO t0 VALUES(1);
SELECT c0 FROM t0 WHERE CASE 0 WHEN 1 THEN ('a')^('b') ELSE 1 END;
SHOW WARNINGS;
```

### 2. What did you expect to see? (Required)
SELECT succeed to query results without throwing warnings.
### 3. What did you see instead (Required)
```sql
mysql> SHOW WARNINGS;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1292 | Truncated incorrect INTEGER value: 'a' |
| Warning | 1292 | Truncated incorrect INTEGER value: 'b' |
+---------+------+----------------------------------------+
2 rows in set (0.00 sec)
```
### 4. What is your TiDB version? (Required)
mysql> SELECT tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()

|
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.4.0
Edition: Community
Git Commit Hash: 55f3b24c1c9f506bd652ef1d162283541e428872
Git Branch: heads/refs/tags/v5.4.0
UTC Build Time: 2022-01-25 08:39:26
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)