Possible problems: function call could be eliminated from where class and only
be evaluated once; function can be evaluated during table and item setup phase which could
cause side effects not to be registered in binlog.

Fixed with introducing func_item_sp::used_tables() returning the correct table_map constant.



mysql-test/r/sp.result:
  results changed
mysql-test/t/sp.test:
  regression test demonstrating that function's returns match where condition
  of the top-level query table.
sql/item_func.h:
  private used_tables() method returning the correct table_bit with meaning the item is not
  a constant