The problem was caused by a merged semi-join, which contained a non-merged
semi-join, which used references to the top-level query in the left_expr.

When moving non-merged semi-join from the subquery to its parent, do not
forget to call fix_after_pullout for its Item_subselect. We need to do
that specifically, because non-merged semi-joins do not have their
IN-equality in the WHERE clause at this stage.