After review fixes for Item_equal.


sql/item.cc:
  After review fixes for Item_equal.
sql/item.h:
  After review fixes for Item_equal.
sql/item_cmpfunc.cc:
  After review fixes for Item_equal.
sql/item_cmpfunc.h:
  After review fixes for Item_equal.
sql/item_func.h:
  After review fixes for Item_equal.
sql/item_func.cc:
  After review fixes for Item_equal.
sql/item_row.cc:
  After review fixes for Item_equal.
sql/item_row.h:
  After review fixes for Item_equal.
sql/item_strfunc.h:
  After review fixes for Item_equal.
sql/opt_range.cc:
  After review fixes for Item_equal.
sql/sql_list.h:
  After review fixes for Item_equal.
sql/sql_select.cc:
  After review fixes for Item_equal.
mysql-test/r/select.result:
  After review fixes for Item_equal.
mysql-test/r/subselect.result:
  After review fixes for Item_equal.