Added a test case for bug #9338.
sql_select.cc:
  Fixed bug #9338.
  When an occurence of a field reference has to be replaced
  by another field reference the whole Item_field must be
  replaced.
item.cc:
  Fixed bug #9338.
  The method Item_field::replace_equal_field_processor was
  replaced by Item_field::replace_equal_field. The new method
  is used to replace the occurences of Item_field objects.
item.h:
  Fixed bug #9338.
  The virtual function replace_equal_field_processor was replaced
  by replace_equal_field. The latter is supposed to be used as a
  callback function in calls of the method transform.