Attempt 3: SkRasterPipelineBlitter: support A8

Now that SkOpts_hsw.cpp no longer hooks in SkRasterPipeline_opts,
it should be safe to try this again.

This reverts commit 86d55b312a2649d80890ccf75f24571ada0265f1.

Change-Id: I2d495600ca9d3a0f49c2e02fbaaae349cefac3a1
Reviewed-on: https://skia-review.googlesource.com/6985
Reviewed-by: Mike Klein <mtklein@chromium.org>
6 files changed