Remove detach_secondary_results() and other cleanups.
Remove detach_secondary_results() and other cleanups.

- The detach_secondary_results() is a leftover from the two-plane value
  representation. Use detach_results() instead to remove all instruction
  results.
- Make the append_* DFG methods more direct. Don't depend on calling the
  corresponding attach_* methods. Just create a new value directly,
  using the values.next_key() trick.

sunfishcode/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne,stoklund/cretonne