Reland "more JIT refactoring"

This is a reland of 558b6392250e6dd4a92dc4a94640c7ec2cda66e7

PS2... oh, right, not everything supports AVX2.

Original change's description:
> more JIT refactoring
>
> This re-enables AVX2 JIT with simultaneous register assignment and
> instruction selection.  You can see it working in a very basic way in
> how we choose instructions and registers for Op::mad_f32.
>
> Constants are still broadcast, here inside the loop instead of hoisted.
> I think it'll probably end up best to use constants directly from memory
> (as in vpshufb's masks), falling back to these in-loop broadcasts when
> that can't work.
>
> Change-Id: If17d51b9960f08da3612e51ac04424e996bf83d4
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228366
> Commit-Queue: Mike Klein <mtklein@google.com>
> Reviewed-by: Mike Klein <mtklein@google.com>

Cq-Include-Trybots: skia.primary:Test-Mac10.13-Clang-VMware7.1-CPU-AVX-x86_64-Debug-All-NativeFonts
Change-Id: I6f99d275040abe6210a980fc544f7f22c3b85727
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228476
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed