optimize the list columns partition pruner

[crazycs520]

1. Minimal reproduce step (Required)

create table t (a bigint, b int) partition by list columns (a,b) (partition p0 values in ((1,1),(2,2)), partition p1 values in ((3,3)));

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

test> desc select * from t where a=1;
+------------------------+----------+-----------+-----------------------+--------------------------------+
| id                     | estRows  | task      | access object         | operator info                  |
+------------------------+----------+-----------+-----------------------+--------------------------------+
| PartitionUnion_8       | 20.00    | root      |                       |                                |
| ├─TableReader_11       | 10.00    | root      |                       | data:Selection_10              |
| │ └─Selection_10       | 10.00    | cop[tikv] |                       | eq(test.t.a, 1)                |
| │   └─TableFullScan_9  | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
+------------------------+----------+-----------+-----------------------+--------------------------------+

3. What did you see instead (Required)

test> desc select * from t where a=1;
+------------------------+----------+-----------+-----------------------+--------------------------------+
| id                     | estRows  | task      | access object         | operator info                  |
+------------------------+----------+-----------+-----------------------+--------------------------------+
| PartitionUnion_8       | 20.00    | root      |                       |                                |
| ├─TableReader_11       | 10.00    | root      |                       | data:Selection_10              |
| │ └─Selection_10       | 10.00    | cop[tikv] |                       | eq(test.t.a, 1)                |
| │   └─TableFullScan_9  | 10000.00 | cop[tikv] | table:t, partition:p0 | keep order:false, stats:pseudo |
| └─TableReader_14       | 10.00    | root      |                       | data:Selection_13              |
|   └─Selection_13       | 10.00    | cop[tikv] |                       | eq(test.t.a, 1)                |
|     └─TableFullScan_12 | 10000.00 | cop[tikv] | table:t, partition:p1 | keep order:false, stats:pseudo |
+------------------------+----------+-----------+-----------------------+--------------------------------+

4. What is your TiDB version? (Required)

> select tidb_version();
+-------------------------------------------------------------------+
| tidb_version()                                                    |
+-------------------------------------------------------------------+
| Release Version: v4.0.0-beta.2-1676-g3e073c8a0                    |
| Edition: Community                                                |
| Git Commit Hash: 3e073c8a0878db6ab85ce255ad7db804e35d5225         |
| Git Branch: master                                                |
| UTC Build Time: 2020-11-30 08:57:47                               |
| GoVersion: go1.14.4                                               |
| Race Enabled: false                                               |
| TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306 |
| Check Table Before Drop: false                                    |
+-------------------------------------------------------------------+