restore sRGB memset optimization

restore sRGB memset optimization

https://skia-review.googlesource.com/c/5275/ removed it, and perf noticed.

This is obviously not very pretty or scalable.  I plan to folow up with a more thorough and principled way to do this sort of constant-color + invariant-stage == constant-color optimization.

BUG=skia:6013

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I377386f67e66169cce6e0cb0831f3b7154496840
Reviewed-on: https://skia-review.googlesource.com/5338
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>