Don't use the Sk4f gradient impl without SIMD

src/opts/SkNx_avx.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkNx_avx_DEFINED
 #define SkNx_avx_DEFINED
 
 // This file may assume <= AVX, but must check SK_CPU_SSE_LEVEL for anything more recent.
 
 // All the SSE specializations are still good ideas.  We'll just add Sk8f.
 #include "SkNx_sse.h"
 
+// SkNx_sse.h defines SKNX_IS_FAST.
+
 namespace {  // See SkNx.h
 
 template <>
 class SkNx<8, float> {
 public:
     SkNx(const __m256& vec) : fVec(vec) {}
 
     SkNx() {}
     SkNx(float val) : fVec(_mm256_set1_ps(val)) {}
     static SkNx Load(const float vals[8]) { return _mm256_loadu_ps(vals); }
@@ skipping 55 matching lines @@
     SkNx thenElse(const SkNx& t, const SkNx& e) const {
         return _mm256_blendv_ps(e.fVec, t.fVec, fVec);
     }
 
     __m256 fVec;
 };
 
 }  // namespace
 
 #endif//SkNx_avx_DEFINED