Don't use the Sk4f gradient impl without SIMD

src/effects/gradients/SkLinearGradient.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLinearGradient.h"
 
 static const float kInv255Float = 1.0f / 255;
@@ skipping 264 matching lines @@
     } while (--count != 0);
 }
 
 }
 
 void SkLinearGradient::LinearGradientContext::shadeSpan(int x, int y, SkPMColor* SK_RESTRICT dstC,
                                                         int count) {
     SkASSERT(count > 0);
     const SkLinearGradient& linearGradient = static_cast<const SkLinearGradient&>(fShader);
 
-#ifndef SK_SUPPORT_LEGACY_LINEAR_GRADIENT_TABLE
+// Only use the Sk4f impl when known to be fast.
+#if defined(SKNX_IS_FAST)
     if (SkShader::kClamp_TileMode == linearGradient.fTileMode &&
         kLinear_MatrixClass == fDstToIndexClass)
     {
         this->shade4_clamp(x, y, dstC, count);
         return;
     }
 #endif
 
     SkPoint             srcPt;
     SkMatrix::MapXYProc dstProc = fDstToIndexProc;
@@ skipping 619 matching lines @@
         }
     } else {
         if (fApplyAlphaAfterInterp) {
             this->shade4_dx_clamp<true, false>(dstC, count, fx, dx, invDx, dither);
         } else {
             this->shade4_dx_clamp<false, false>(dstC, count, fx, dx, invDx, dither);
         }
     }
 }