wrong aggEliminateCheck for select group_concat() group by unique

[XuHuaiyu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table t(id int primary key, a int, b int);
insert into t values(1,1,1);
insert into t values(2,1,1);
select group_concat(a,b) from t group by id;

2. What did you expect to see?

+-------------------+
| group_concat(a,b) |
+-------------------+
| 11                |
| 11                |
+-------------------+

3. What did you see instead?

+-------------------+
| group_concat(a,b) |
+-------------------+
| 1                 |
| 1                 |
+-------------------+

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

Release Version: v3.0.0-beta.1-8-g7cabaac61
Git Commit Hash: 7cabaac61dbc7fa65948ee7fbd11018af7765369
Git Branch: master
UTC Build Time: 2019-03-27 03:25:00
GoVersion: go version go1.12 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false