Bug #59943 materialization=on/off leads to different result set when using IN & VAR_POP

Description:
SELECT * FROM t2 WHERE (X) IN (SELECT VAR_POP(`id`) AS X FROM C);
Returns different results (using a InnoDB, MyISAM or Memory table) when materialization is turned on or off.

How to repeat:
DROP DATABASE IF EXISTS test2; CREATE DATABASE test2; USE test2; /* Test setup */

CREATE TABLE C (id int);
INSERT INTO C VALUES (5),(9),(9);

CREATE TABLE t2 SELECT VAR_POP(`id`) AS X FROM C;

SET @@SESSION.optimizer_switch="materialization=off";

SELECT * FROM t2 WHERE (X) IN (SELECT VAR_POP(`id`) AS X FROM C);  /* 3.5556 */

SET @@SESSION.optimizer_switch="materialization=on";

SELECT * FROM t2 WHERE (X) IN (SELECT VAR_POP(`id`) AS X FROM C);  /* No results */

Suggested fix:
Return same results.

See also bug #59833

Noted in 5.6.6 changelog. 

IN subqueries that used a variance or standard deviation aggregate 
function could return a different result depending on whether the 
optimizer_switch materialization flag was enabled.