mysql-test/r/subselect.result:
  results of new optimisation
mysql-test/t/subselect.test:
  results of new optimisation
sql/item_cmpfunc.h:
  function type
sql/item_func.h:
  function type
sql/item_subselect.cc:
  new optimisation for ref_or_null
sql/item_subselect.h:
  new optimisation for ref_or_null
sql/sql_select.cc:
  new optimisation for ref_or_null
sql/sql_select.h:
  item_subselect.cc need safe_index_read