div255(x) as ((x+128)*257)>>16 with SSE

src/core/Sk4px.h

Index: src/core/Sk4px.h
diff --git a/src/core/Sk4px.h b/src/core/Sk4px.h
index a7f5c9f4c6fcc11e6962ef3121e8a9ed79db3d3c..3755488a4a21274fd44f8ee3d89a6a92db581791 100644
--- a/src/core/Sk4px.h
+++ b/src/core/Sk4px.h
@@ -66,11 +66,7 @@ public:
         Sk4px addNarrowHi(const Sk16h&) const;
 
         // Rounds, i.e. (x+127) / 255.
-        Sk4px div255() const {
-            // Calculated as ((x+128) + ((x+128)>>8)) >> 8.
-            auto v = *this + Sk16h(128);
-            return v.addNarrowHi(v >> 8);
-        }
+        Sk4px div255() const;
 
         // These just keep the types as Wide so the user doesn't have to keep casting.
         Wide operator * (const Wide& o) const { return INHERITED::operator*(o); }