The problem here occurs when a subquery's query block is removed
during transformation of an IN subquery or a scalar subquery, and
semi-join is not applicable, and Item::remove_dependence_processor()
is used to modify items below the subquery to be removed.

One such item is an Item_direct_ref which is added in the
transformation of an IN subquery, see e.g
Item_in_subselect::single_value_transformer(). The Item_direct_ref
may be created with a dependent_from field pointing to the subquery
to be removed. In this case, dependent_from most be moved to the outer query block, but remove_dependence_processor() fails to handle this situation.

Luckily, Item_ident::fix_after_pullout() does exactly this, when first
making sure that the name resolution contexts of the eliminated query
block are merged with its parent.

The code snippet that merges name resolution contexts is made into a
new member function of SELECT_LEX, named merge_contexts().

Item::remove_dependence_processor() was also deleted.