Don't use the Sk4f gradient impl without SIMD

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index f47b6ab30e7fa81392abd02afe9a5a89f38d687c..22683033e81096f8e63f0eeb0a0d5319a1468416 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -282,7 +282,8 @@ void SkLinearGradient::LinearGradientContext::shadeSpan(int x, int y, SkPMColor*
     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)
     {