The optimizer sets index traversal in reverse order only if there are 
used key parts that are not compared to a constant.
However using the primary key as an ORDER BY suffix rendered the check
incomplete : going in reverse order must still be used even if 
all the parts of the secondary key are compared to a constant.

Fixed by relaxing the check and set reverse traversal even when all
the secondary index keyparts are compared to a const.
Also account for the case when all the primary keys are compared to a
constant.


mysql-test/include/mix1.inc:
  Bug #31001: test case
mysql-test/r/innodb_mysql.result:
  Bug #31001: test case
sql/sql_select.cc:
  Bug #31001: 
   - account for the case when all the primary key parts are compared
     to a constant.
   - force test_if_skip_sort_order to go backwards over the key even 
     when the number of keyparts used is the same as the number of
     keyparts equal to a constant. (because of the primary key
     suffix).