Color dodge and burn with SkPMFloat.

src/opts/SkPMFloat_sse.h

Index: src/opts/SkPMFloat_sse.h
diff --git a/src/opts/SkPMFloat_sse.h b/src/opts/SkPMFloat_sse.h
index 802b17ba0cc6e38b7ced3f4f45f0bcda1544929a..28aa90bf29fd8d4913417f8111d92f1bc9b23448 100644
--- a/src/opts/SkPMFloat_sse.h
+++ b/src/opts/SkPMFloat_sse.h
@@ -33,4 +33,9 @@ inline SkPMColor SkPMFloat::round() const {
     return c;
 }
 
+inline Sk4f SkPMFloat::alphas() const {
+    static_assert(SK_A32_SHIFT == 24, "");
+    return _mm_shuffle_ps(fVec, fVec, 0xff);  // Read as 11 11 11 11, copying lane 3 to all lanes.
+}
+
 }  // namespace