Implement a global (whole function) register allocator.
This replaces the local (per basic block) register allocator.

Clobbering of registers by instructions is handled properly.
A separate change will add the correct clobbers to all the instructions.

Change-Id: I38ce4dc7dccb8303c1c0e0295fe70247b0a3f2ea
Reviewed-on: https://go-review.googlesource.com/13622Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Todd Neal <todd@tneal.org>