value end with space's unique check doesn't works when columns be non-utf8 collation + prefix column index

[lysu]

Bug Report

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1. Minimal reproduce step (Required)

create table zz (x varchar(40) CHARACTER SET ascii COLLATE ascii_bin, UNIQUE KEY `uk`(`x`(4)));
insert into zz select 'abc ';
insert into zz select 'abc d';

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

insert into zz select 'abc d';
mysql> insert into zz select 'abc d';
ERROR 1062 (23000): Duplicate entry 'abc ' for key 'uk'

3. What did you see instead (Required)

mysql> insert into zz select 'abc d';
Query OK, 1 row affected (0.01 sec)
Records: 1  Duplicates: 0  Warnings: 0

4. What is your TiDB version? (Required)

5.0.0, 5.0.1

[ti-srebot]

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