Multiple equalities were not adjusted after reading constant tables.
It resulted in neglecting good index based methods that could be
used to access of other tables.


mysql-test/r/having.result:
  Adjusted a test case results after fix for bug #16504.
mysql-test/r/select.result:
  Added a test case for bug #16504.
mysql-test/r/subselect.result:
  Adjusted a test case results after fix for bug #16504.
mysql-test/r/varbinary.result:
  Adjusted a test case results after fix for bug #16504.
mysql-test/t/select.test:
  Added a test case for bug #16504.
sql/item.cc:
  Fixed bug #16504.
  An Item_equal object may contain only a constant member.
  It may happen after reading constant tables.
sql/item_cmpfunc.cc:
  Fixed bug #16504.
  Added method Item_equal::check_const that check appearance of new 
  constant items in a multiple equality.
sql/item_cmpfunc.h:
  Fixed bug #16504.
  Added method Item_equal::check_const that check appearance of new 
  constant items in a multiple equality.
sql/sql_select.cc:
  Fixed bug #16504.
  Adjusted multiple equalities after reading constant tables.
  Fixed a few typo in comments.