cannot create index on virtual generated column.

[GMHDBJD]

Bug Report

Cannot create index on virtual generated column gened by primary key after #18045 merge.

1. Minimal reproduce step (Required)

drop database if exists `db`;
create database `db`;
use `db`;
create table t1 (id int, PRIMARY KEY (id));
insert into t1 values(1);
alter table t1 add column gen_id int as (id+1);
alter table t1 add index idx(`gen_id`);

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

no error

3. What did you see instead (Required)

ERROR 8202 (HY000): Cannot decode index value, because Miss column

4. Affected version (Required)

Server version: 5.7.25-TiDB-v4.0.0-beta.2-636-gc19018e06

5. Root Cause Analysis

[zimulala]

@wjhuang2016 PTAL