ROR-index_merge relies on Rowid-ordered-retrieval property: a ROR scan,
e.g. a scan on equality range

  tbl.key=const

should return rows ordered by their Rowid. Also, handler->cmp_ref() should
compare rowids according to the Rowid ordering.

When the table's primary key uses DESC keyparts, ROR scans return rows
according to the PK's ordering.

But ha_innobase::cmp_ref() compared rowids as if PK used ASC keyparts.
This caused wrong query results with index_merge.

Fixed this by making ha_innobase::cmp_ref() compare according to the PK
defintion, including keypart's DESC property.