- Turning get_mm_tree_for_const() from a static function into
  a protected method in Item.
- Adding a new class Item_bool_func2_with_rev, for the functions and operators
  that have a reverse function and can use the range optimizer for
  to optimize "value OP field" as "field REV_OP value". Deriving
  Item_bool_rowready_func2 and Item_funt_spatial_rel from the new class.
- Removing Item_bool_func2::have_rev_func().