reordered Item_row class variables to be sorted by memory size


mysql-test/r/subselect.result:
  fix result of subselect test
sql/item.cc:
  postreviews fix
sql/item.h:
  postreviews fix
sql/item_cmpfunc.h:
  postreviews fix
sql/item_row.cc:
  postreviews fix
sql/item_row.h:
  reordered class variables to be sorted by memory size
  postreviews fix
sql/item_strfunc.cc:
  postreviews fix
sql/item_subselect.cc:
  postreviews fix