Remove overly-promiscuous SkNx syntax sugar.

src/effects/gradients/SkRadialGradient.cpp

Index: src/effects/gradients/SkRadialGradient.cpp
diff --git a/src/effects/gradients/SkRadialGradient.cpp b/src/effects/gradients/SkRadialGradient.cpp
index fec180cfa10d4328e2e7fd12f411f2a03d3e427d..b6b31c592f0856ce6d38694c51e06b260fdd123b 100644
--- a/src/effects/gradients/SkRadialGradient.cpp
+++ b/src/effects/gradients/SkRadialGradient.cpp
@@ -320,8 +320,8 @@ void shadeSpan_radial_clamp2(SkScalar sfx, SkScalar sdx, SkScalar sfy, SkScalar
 
         for (int i = 0; i < (count >> 2); ++i) {
             Sk4f dist = Sk4f::Min(fast_sqrt(R), max);
-            R += dR;
-            dR += ddR;
+            R = R + dR;
+            dR = dR + ddR;
 
             int fi[4];
             dist.castTrunc().store(fi);