This prepares us for moving most of the register assignment to the
ProgramBinary class itself, and cleans up the various methods.

BUG=angle:466

Change-Id: Ie2965ed41ac50617584fd7a6d7b0ec2784e7dfaf
Reviewed-on: https://chromium-review.googlesource.com/207252Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>