Add a simple register allocator. It does only intra-basicblock
allocation. It uses a greedy one-pass allocation treating the
register file as a cache.
Change-Id: Ib6b52f48270e08dfda98f2dd842b05afc3ab01ce
Reviewed-on: https://go-review.googlesource.com/9761
Reviewed-by: Alan Donovan <adonovan@google.com>
