rephrase lowp constant_color

This doesn't change the generated code (no .S files change),
but it does rephrase what we're trying to do to make it
generalize to AVX2 better:

   - load 4 floats
   - add 256.0f to each
   - splat out the low 2 bytes of each 4 byte lane as r,g,b,a

Change-Id: Iadc5bc1f2a268679d1ccadd31cd24949a71e0aa4
Reviewed-on: https://skia-review.googlesource.com/20270
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed