Remove overly-promiscuous SkNx syntax sugar.

bench/PMFloatBench.cpp

Index: bench/PMFloatBench.cpp
diff --git a/bench/PMFloatBench.cpp b/bench/PMFloatBench.cpp
index 14ccb33d0a805c74bff9cfc01b7c4ff8c845df06..37542e3525f09059e36211d8eb61e604d35a28bc 100644
--- a/bench/PMFloatBench.cpp
+++ b/bench/PMFloatBench.cpp
@@ -121,10 +121,10 @@ struct PMFloatGradientBench : public Benchmark {
                 fDevice[i+1] = SkPMFloat(b).trunc();
                 fDevice[i+2] = SkPMFloat(c).trunc();
                 fDevice[i+3] = SkPMFloat(d).trunc();
-                a += dcdx4;
-                b += dcdx4;
-                c += dcdx4;
-                d += dcdx4;
+                a = a + dcdx4;
+                b = b + dcdx4;
+                c = c + dcdx4;
+                d = d + dcdx4;
             }
         }
     }