bug of select on general column when add generated column first.

[crazycs520]

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 t (a int, b int as (a + 1), c int as (b + 1))
insert into t set a=1;
alter table t add column e int as (c + 1) first;  -- this will get error in mysql: 'Generated column can refer only to generated columns defined prior to it.'
select * from t
+--------+---+---+---+
| e      | a | b | c |
+--------+---+---+---+
| <null> | 1 | 2 | 3 |                      
+--------+---+---+---+
1 row in set
desc select * from t;
+-----------------+----------+------+-------------------------------------------------------------------------------------------------------------------+
| id              | count    | task | operator info                                                                                                     |
+-----------------+----------+------+-------------------------------------------------------------------------------------------------------------------+
| Projection_5    | 10000.00 | root | cast(plus(cast(plus(test.t.b, 1)), 1)), test.t.a, cast(plus(test.t.a, 1)), cast(plus(cast(plus(test.t.a, 1)), 1)) |
| └─TableReader_7 | 10000.00 | root | data:TableScan_6                                                                                                  |
|   └─TableScan_6 | 10000.00 | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo                                                        |
+-----------------+----------+------+-------------------------------------------------------------------------------------------------------------------+

2. What did you expect to see?

mysql>alter table t add column e int as (c + 1) first;
(3107, u'Generated column can refer only to generated columns defined prior to it.')

3. What did you see instead?

alter table t add column e int as (c + 1) first; 
--success
select * from t
+--------+---+---+---+
| e      | a | b | c |
+--------+---+---+---+
| <null> | 1 | 2 | 3 |                          -- the result of e should not null.
+--------+---+---+---+

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

▶ bin/tidb-server -V
Release Version: v3.0.0-rc.1-360-g85de5df03
Git Commit Hash: 85de5df03fd47f80b90c36a63b7d213aeda4468b
Git Branch: master
UTC Build Time: 2019-07-22 06:40:48
GoVersion: go version go1.12 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false