plan: support subquery in Do statement

[eurekaka]

What problem does this PR solve?

Before this PR:

mysql> select * from t1;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.01 sec)

mysql> select 1 in (select * from t1);
+-------------------------+
| 1 in (select * from t1) |
+-------------------------+
|                       1 |
+-------------------------+
1 row in set (0.00 sec)

mysql> do 1 in (select * from t1);
ERROR 2013 (HY000): Lost connection to MySQL server during query

while in MySQL:

mysql> select * from t1;
+--------------+
| processor_id |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

mysql> do 1 in (select * from t1);
Query OK, 0 rows affected (0.00 sec)

MySQL states in https://dev.mysql.com/doc/refman/5.7/en/do.html that:

DO only executes expressions. It cannot be used in all cases where SELECT can be used. For example, DO id FROM t1 is invalid because it references a table.

The documentation does not explicitly specify the boundary to which Do is compatible with SELECT. Since this case experimentally works in MySQL, we support it in TiDB.

What is changed and how it works?

• involve full expression rewriting in plan building for Do;
• prevent Projection specially added for Do being eliminated in optimizer;

Check List

Tests

• Unit test

Side effects

N/A

Related changes

• Need to cherry-pick to the release branch

---

This change is 

[eurekaka]

/run-all-tests

[eurekaka]

@zz-jason @winoros @lamxTyler PTAL

[alivxxx]

Why panic?

[eurekaka]

Because if idx == -1, in line 40, used[idx] = true would panic also, to make the error message in log clearer, I made this change.

[alivxxx]

LGTM

[zz-jason]

LGTM