i965: Replace HW_REG with ARF/FIXED_GRF.

i965: Replace HW_REG with ARF/FIXED_GRF.

HW_REGs are (were!) kind of awful. If the file was HW_REG, you had to
look at different fields for type, abs, negate, writemask, swizzle, and
a second file. They also caused annoying problems like immediate sources
being considered scheduling barriers (commit 6148e94e2) and other such
nonsense.

Instead use ARF/FIXED_GRF/MRF for fixed registers in those files.

After a sufficient amount of time has passed since "GRF" was used, we
can rename FIXED_GRF -> GRF, but doing so now would make rebasing awful.

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>