- Changing Comp_creator::create() and create_swap() to return
  Item_bool_rowready_func2 instead of Item_bool_func2, as they
  can never return neither Item_func_like nor Item_func_xor

- Changing the first argument of Comp_create::create() and create_swap()
  from THD to MEM_ROOT, so the method implementations can now reside in
  item_cmpfunc.h instead of item_cmpfunc.cc and thus make the code slightly
  easier to read.