add 'a8' config for nanobench, specialize blitV for raster-pipeline

Motivated by wanting to speed-up A8 blits in general (and at the moment, aarect blits). More to come in these areas.

Bug: skia:
Change-Id: I45e8ef951b8e89a825af72b1918049be10920137
Reviewed-on: https://skia-review.googlesource.com/39401
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index ab7c689..95977c5 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -475,6 +475,8 @@
     CPU_CONFIG(nonrendering, kNonRendering_Backend,
                kUnknown_SkColorType, kUnpremul_SkAlphaType, nullptr)
 
+    CPU_CONFIG(a8, kRaster_Backend,
+               kAlpha_8_SkColorType, kPremul_SkAlphaType, nullptr)
     CPU_CONFIG(8888, kRaster_Backend,
                kN32_SkColorType, kPremul_SkAlphaType, nullptr)
     CPU_CONFIG(565,  kRaster_Backend,