Implement four more xfermodes with Sk4px.

src/opts/SkXfermode_opts_SSE2.cpp

Index: src/opts/SkXfermode_opts_SSE2.cpp
diff --git a/src/opts/SkXfermode_opts_SSE2.cpp b/src/opts/SkXfermode_opts_SSE2.cpp
index b92477094b72cbd8b6ab5b8f884355bf2f969990..f8772808a790337a3c93549d3202128c91ff2da4 100644
--- a/src/opts/SkXfermode_opts_SSE2.cpp
+++ b/src/opts/SkXfermode_opts_SSE2.cpp
@@ -515,15 +515,17 @@ void SkSSE2ProcCoeffXfermode::toString(SkString* str) const {
 SkProcCoeffXfermode* SkPlatformXfermodeFactory_impl_SSE2(const ProcCoeff& rec,
                                                          SkXfermode::Mode mode) {
     SkXfermodeProcSIMD proc = nullptr;
-    // TODO(mtklein): implement these Sk4px.
     switch (mode) {
+        // TODO(mtklein): Sk4pxXfermode has these now.  Clean up.
         case SkProcCoeffXfermode::kOverlay_Mode:    proc =    overlay_modeproc_SSE2; break;
         case SkProcCoeffXfermode::kDarken_Mode:     proc =     darken_modeproc_SSE2; break;
         case SkProcCoeffXfermode::kLighten_Mode:    proc =    lighten_modeproc_SSE2; break;
-        case SkProcCoeffXfermode::kColorDodge_Mode: proc = colordodge_modeproc_SSE2; break;
-        case SkProcCoeffXfermode::kColorBurn_Mode:  proc =  colorburn_modeproc_SSE2; break;
         case SkProcCoeffXfermode::kHardLight_Mode:  proc =  hardlight_modeproc_SSE2; break;
+
+        // TODO(mtklein): implement these with SkPMFloat.
         case SkProcCoeffXfermode::kSoftLight_Mode:  proc =  softlight_modeproc_SSE2; break;
+        case SkProcCoeffXfermode::kColorDodge_Mode: proc = colordodge_modeproc_SSE2; break;
+        case SkProcCoeffXfermode::kColorBurn_Mode:  proc =  colorburn_modeproc_SSE2; break;
         default: break;
     }
     return proc ? SkNEW_ARGS(SkSSE2ProcCoeffXfermode, (rec, mode, (void*)proc)) : nullptr;