Range scan in descending order for c <= <col> <= c type of
ranges was ignoring the DESC flag.
However some engines like InnoDB have the primary key parts 
as a suffix for every secondary key.
When such primary key suffix is used for ordering ignoring 
the DESC is not valid.
But we generally would like to do this because it's faster.
            
Fixed by performing only reverse scan if the primary key is used.
Removed some dead code in the process.

mysql-test/r/innodb_mysql.result:
  Bug#37830 : test case
mysql-test/t/innodb_mysql.test:
  Bug#37830 : test case
sql/opt_range.cc:
  Bug#37830 : 
  - preserve and use used_key_parts to
    distinguish when a primary key suffix is used
  - removed some dead code
sql/opt_range.h:
  Bug#37830 : 
  - preserve used_key_parts
  - dead code removed
sql/sql_select.cc:
  Bug#37830 : Do only reverse order traversal
  if the primary key suffix is used.