Step#2:

1. Removes the function build_equal_items_for_cond() and
   introduces a new method Item::build_equal_items() instead,
   with specific implementations in the following Items:

   Item  (the default implementation)
   Item_ident_or_func_or_sum
   Item_cond
   Item_cond_and

2. Adds a new abstract class Item_ident_or_func_or_sum,
   a common parent for Item_ident and Item_func_or_sum,
   as they have exactly the same build_equal_items().

3. Renames Item_cond_and::cond_equal to Item_cond_and::m_cond_equal,
   to avoid confusion between the member and local variables named
   "cond_equal".