plan: make USE INDEX(PRIMARY) works on the integer primary key

[zz-jason]

What have you changed? (mandatory)

Take this as an example:

TiDB(localhost:4000) > create table t(a bigint primary key);
Query OK, 0 rows affected (0.11 sec)

TiDB(localhost:4000) > select * from t use index(`primary`) where a = 1;
ERROR 1176 (42000): Key 'primary' doesn't exist in table 't'

TiDB(localhost:4000) > explain select * from t ignore index(`primary`) where a = 1;
ERROR 1176 (42000): Key 'primary' doesn't exist in table 't'

This PR fixes this issue, makes the use index(primary) works when the primary key is an integer column. After this PR:

TiDB(localhost:4000) > explain select * from t use index(`primary`) where a = 1;
+-------------------+-------+------+------------------------------------------------------+
| id                | count | task | operator info                                        |
+-------------------+-------+------+------------------------------------------------------+
| TableReader_6     | 1.00  | root | data:TableScan_5                                     |
| └─TableScan_5     | 1.00  | cop  | table:t, range:[1,1], keep order:false, stats:pseudo |
+-------------------+-------+------+------------------------------------------------------+
2 rows in set (0.01 sec)

TiDB(localhost:4000) > explain select * from t ignore index(`primary`) where a = 1;
+-------------------+-------+------+------------------------------------------------------+
| id                | count | task | operator info                                        |
+-------------------+-------+------+------------------------------------------------------+
| TableReader_6     | 1.00  | root | data:TableScan_5                                     |
| └─TableScan_5     | 1.00  | cop  | table:t, range:[1,1], keep order:false, stats:pseudo |
+-------------------+-------+------+------------------------------------------------------+
2 rows in set (0.00 sec)

What is the type of the changes? (mandatory)

• Bug fix (non-breaking change which fixes an issue)

How has this PR been tested? (mandatory)

• existing tests
• newly added unit test

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

No

Does this PR affect tidb-ansible update? (mandatory)

No

Does this PR need to be added to the release notes? (mandatory)

release note:
Fix the the primary key doesn't exist error when the primary key is the table handle.

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

[zz-jason]

/run-all-tests

[winkyao]

LGTM

[coocood]

LGTM

[zz-jason]

@coocood @winoros PTAL

[jackysp]

/run-all-tests