wrong results when comparing bit with string

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table xx (b bit(1));
insert into xx values (b'1');
select count(*) from xx where b='1';

The reason is that we select a wrong target field type in function RefineComparedConstant.

2. What did you expect to see?

mysql> create table xx (b bit(1));
Query OK, 0 rows affected (0.05 sec)

mysql> insert into xx values (b'1');
Query OK, 1 row affected (0.00 sec)

mysql> select count(*) from xx where b='1';
+----------+
| count(*) |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> create table xx (b bit(1));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into xx values (b'1');
Query OK, 1 row affected (0.00 sec)

mysql> select count(*) from xx where b='1';
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

mysql> select version();
+----------------------------------------+
| version()                              |
+----------------------------------------+
| 5.7.25-TiDB-v4.0.0-alpha-32-g45790b579 |
+----------------------------------------+
1 row in set (0.00 sec)