sql/ha_heap.h:
  Move HA_NOT_READ_PREFIX_LAST to index_flags()
sql/ha_isammrg.h:
  Move HA_NOT_READ_PREFIX_LAST to index_flags()
sql/handler.h:
  Added comment
sql/sql_select.cc:
  Test HA_NOT_READ_PREFIX_LAST from index_flags() instead of table_flags().
  Should fix some ORDER BY ... DESC problems with InnoDB