Archival checkin (won't build): monster refactor; don't allocate vregs at

Archival checkin (won't build): monster refactor; don't allocate vregs at
instruction selection time, but instead deal with 'values' which are tied to
their parent IR. The code is now way cleaner and more understandable. We'll
aasign vregs in a later pass, as we convert to elementary form.