fixed number of rows of external field reported to optimizer
added check of choosen key (checked left expression tag)
(SCRUM fix for simple IN optimisation)


mysql-test/r/subselect.result:
  new EXPLAIN results ufter fixing used_tables() of Item_cache
sql/item_cmpfunc.cc:
  fixed used_tables asignment
sql/item_subselect.cc:
  added left expression referenca tag
sql/item_subselect.h:
  fixed layout
sql/mysql_priv.h:
  left expression reference tag
sql/mysqld.cc:
  left expression reference tag
sql/sql_select.cc:
  checked left expression reference tag
  fixed number of rows in outer reference (it should be constant)