Checks in `Preprocess` doesn't work with `PREPARE/EXECUTE`

## Bug Report

### 1. Minimal reproduce step (Required)

Case 1

```sql
create table t(a int primary key, b int);
select @@tidb_enable_shared_lock_promotion;
select * from t where a = 1 for share; -- expect fail
set @@tidb_enable_shared_lock_promotion = 1;
select * from t where a = 1 for share; -- expect success
prepare stmt from 'select * from t where a = 1 for share';
execute stmt; -- expect success
set @@tidb_enable_shared_lock_promotion = 0;
execute stmt; -- expect fail
select * from t where a = 1 for share; -- expect fail
```

Case 2

```sql
set sql_mode='oracle';
select count(*) from t t1 join t t1; -- expect success
prepare stmt1 from 'select count(*) from t t1 join t t1';
set sql_mode=default;
execute stmt1; -- expect fail
select count(*) from t t1 join t t1; -- expect fail
```

Case 3 (Only on the feature/active-active feature branch)

```sql
create table t1 (id int primary key, name varchar(50)) softdelete retention 7 day;
prepare stmt from 'select id from t1 where _tidb_softdelete_time > ?'; -- expect fail
set @@tidb_translate_softdelete_sql = false;
prepare stmt from 'select id from t1 where _tidb_softdelete_time > ?'; -- expect success
set @a = now();
execute stmt using @a; -- expect success
set tidb_translate_softdelete_sql = true;
execute stmt using @a; -- expect fail
```

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

As said above.

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

After `PREPARE` and then modifying system variables, checks in `Preprocess` won't be triggered again in `EXECUTE`.

Note that this issue is related to `PREPARE/EXECUTE` itself, not the plan cache. (See case 2)

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

master, v8.5