Add AVX2 implementation for blit_row_s32a_opaque

The function is added in SkOpts_hsw but doesn't have a AVX2 implementation.
The implementation boosts Vellamo Pixelblender test case for 20% performance

Change-Id: I3bf77eb7629213df1f1bdfa1087ebaf40894d7c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215400
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed