performance degradation.

Filesort + join cache combination is preferred to full index scan because it
is usually faster. But it's not the case when the index is clustered one.

Now test_if_skip_sort_order function prefers filesort only if index isn't
clustered.

mysql-test/r/innodb_mysql.result:
  Added a test case for the bug#50843.
mysql-test/t/innodb_mysql.test:
  Added a test case for the bug#50843.
sql/sql_select.cc:
  Bug#50843: Filesort used instead of clustered index led to
  performance degradation.
  Now test_if_skip_sort_order function prefers filesort only if index isn't
  clustered.