- More testcases
- Let add_ft_key() set keyuse->usable

mysql-test/r/table_elim.result:
  MWL#17: Table elimination
  - More testcases
mysql-test/t/table_elim.test:
  MWL#17: Table elimination
  - More testcases
sql/sql_select.cc:
  MWL#17: Table elimination
  - Let add_ft_key() set keyuse->usable