force SkRasterPipeline when using complex blends

Looks like we're faster than legacy pretty much across the board.

out/nanobench -m Hue Saturation Luminosity Color\$ Color_aa\$ --config 8888 -q --ms 2000

    Xfermode_Luminosity      80128.68 -> 27189.83
    Xfermode_Luminosity_aa   18001.46 -> 14449.67
    Xfermode_Color          183520.09 -> 27448.49
    Xfermode_Color_aa        34257.75 -> 14694.85
    Xfermode_Saturation      80323.96 -> 36449.01
    Xfermode_Saturation_aa   19605.81 -> 18576.62
    Xfermode_Hue            125866.82 -> 36611.03
    Xfermode_Hue_aa          25318.00 -> 18489.33

Change-Id: I842b8f53fb356bf1a5e81502be88e4b34d4ad0dc
Reviewed-on: https://skia-review.googlesource.com/17992
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

Author: Mike Klein

src/core/SkBlitter.cpp

@@ -784,12 +784,16 @@ bool SkBlitter::UseRasterPipelineBlitter(const SkPixmap& device, const SkPaint&
     if (device.colorSpace()) {
         return true;
     }
-
     // ... unless the shader is raster pipeline-only.
     if (paint.getShader() && as_SB(paint.getShader())->isRasterPipelineOnly()) {
         return true;
     }
-
+    #ifndef SK_SUPPORT_LEGACY_RASTERPIPELINE
+    // ... or unless the blend mode is complicated enough.
+    if (paint.getBlendMode() > SkBlendMode::kLastSeparableMode) {
+        return true;
+    }
+    #endif
     return device.colorType() != kN32_SkColorType;
 #endif
 }