use partial primary key if another index can prevent filesort

The fix for bug #28404 causes the covering ordering indexes to be 
preferred unconditionally over non-covering and ref indexes.

Fixed by comparing the cost of using a covering index to the cost of
using a ref index even for covering ordering indexes.
Added an assertion to clarify the condition the local variables should
be in.

mysql-test/include/mix1.inc:
  Bug #36259: fixed a non-stable test case
mysql-test/r/innodb_mysql.result:
  Bug #36259 and #45828 : test case
mysql-test/t/innodb_mysql.test:
  Bug #36259 and #45828 : test case
sql/sql_select.cc:
  Bug #36259 and #45828 : don't consider covering indexes supperior to
  ref keys.