MDEV-16727: Server crashes in Item_equal_iterator<List_iterator_fast, Item>::get_curr_field()

The bug appeares because of the lamely saved list of multiple equalities.
To fix it and_new_conditions_to_optimized_cond() was changed.

mysql-test/main/in_subq_cond_pushdown.test

@@ -792,3 +792,32 @@ WHERE tbl.a IN
 );
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-16727: failure assertion caused by the lamely saved list
+--echo #             of multiple equalities
+--echo #
+
+CREATE TABLE t1 (a varchar(1));
+INSERT INTO `t1` VALUES ('x'), ('y'), ('z');
+
+CREATE TABLE t2 (b varchar(1));
+INSERT INTO t2 VALUES ('x');
+
+CREATE TABLE t3 (c varchar(1));
+INSERT INTO t3 VALUES ('y');
+
+CREATE TABLE t4 (d varchar(1));
+INSERT INTO t4 VALUES ('x'), ('z');
+
+SELECT * FROM t1
+JOIN t2 ON (t1.a=t2.b)
+LEFT JOIN t3 ON (t1.a=t3.c)
+WHERE (t1.a) IN
+(
+  SELECT t4.d
+  FROM t4
+  ORDER BY t4.d
+);
+
+DROP TABLE t1,t2,t3,t4;