Remove overly-promiscuous SkNx syntax sugar.

bench/PMFloatBench.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Benchmark.h"
 #include "SkPMFloat.h"
 
@@ skipping 103 matching lines @@
         for (int n = 0; n < loops; n++) {
             Sk4f a = c0 + dc*fx + Sk4f(0.5f),  // The +0.5f lets us call trunc() instead of get().
                  b = a + dcdx,
                  c = b + dcdx,
                  d = c + dcdx;
             for (size_t i = 0; i < SK_ARRAY_COUNT(fDevice); i += 4) {
                 fDevice[i+0] = SkPMFloat(a).trunc();
                 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;
             }
         }
     }
 };
 
 DEF_BENCH(return new PMFloatGradientBench;)