MDEV-29294 Assertion `functype() == ((Item_cond *) new_item)->functype()' failed in Item_cond::remove_eq_conds on SELECT

Item_singlerow_subselect may be converted to Item_cond during
optimization. So there is a possibility of constructing nested
Item_cond_and or Item_cond_or which is not allowed (such
conditions must be flattened).
This commit checks if such kind of optimization has been applied
and flattens the condition if needed