[Patch idea by Igor Babaev]

Symptom: for IN (SELECT ...) subqueries using IN-to-EXISTS transformation,
the optimizer was unable to make inferences using multiple equalities.

The cause is code Item_in_subselect::inject_in_to_exists_cond() which may
break invariants that Multiple-Equality code relies on. In particular, it
may produce a WHERE condition with an empty Item_cond::m_cond_equal.

Fixed this by making Item_cond::m_cond_equal.