Unexpected results caused by column pruning rules

[bb7133]

Bug Report

1. What did you do?
   Run SQLs like the following:

mysql> select 1 from (select sleep(1)) t;

2. What did you expect to see?

mysql> select 1 from (select sleep(1)) t;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (1.00 sec)

3. What did you see instead?

mysql> select 1 from (select sleep(1)) t;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

Notice the execution time, sleep(1) is not executed.

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

sh> tidb-server -V
Release Version: v4.0.0-alpha-142-g932c4a7bf-dirty
Git Commit Hash: 932c4a7bfc3ad448a2d4c8d68acbb0264d9a588a
Git Branch: master
UTC Build Time: 2019-08-28 06:20:44
GoVersion: go version go1.12 linux/amd64
Race Enabled: false
TiKV Min Version: v3.0.0-60965b00