Simplify (and robustify) regular PACKing

* Make sure that only wired TNs are allocated to stack locations
  until the final pass.

* When using loop depth to prioritise TNs, find them by iterating over
  the list of TNs, instead of reprocessing global TNs that appear in
  multiple IR2 blocks.

* In the final pass that allocates leftover TNs (mostly to the stack),
  make sure that any TN with a negative spill cost (i.e., the score
  says it's actually useful to spill it) is allocated a stack slot.