Speed up centered hard stop shader

src/effects/gradients/SkGradientShader.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fLinearGradient.h"
 #include "SkGradientShaderPriv.h"
 #include "SkLinearGradient.h"
@@ skipping 1098 matching lines @@
                 fragBuilder->codeAppendf("        clamp_t = 1.0 - fract(%s);", t);
                 fragBuilder->codeAppendf("    }");
                 fragBuilder->codeAppendf("}");
             }
 
             // Calculate color
             fragBuilder->codeAppendf("float relative_t = fract(2.0 * clamp_t);");
             if (SkShader::kClamp_TileMode == ge.fTileMode) {
                 fragBuilder->codeAppendf("relative_t += step(1.0, %s);", t);
             }
-            fragBuilder->codeAppendf("vec4 colorTemp = mix(%s[0], %s[1], relative_t);", colors,
-                                     colors);
+
+            fragBuilder->codeAppendf("vec4 start = %s[0];", colors);
+            fragBuilder->codeAppendf("vec4 end   = %s[1];", colors);
             fragBuilder->codeAppendf("if (clamp_t >= 0.5) {");
-            fragBuilder->codeAppendf("    colorTemp = mix(%s[2], %s[3], relative_t);", colors,
-                                     colors);
+            fragBuilder->codeAppendf("    start = %s[2];", colors);
+            fragBuilder->codeAppendf("    end   = %s[3];", colors);
             fragBuilder->codeAppendf("}");
+            fragBuilder->codeAppendf("vec4 colorTemp = mix(start, end, relative_t);");
 
             if (GrGradientEffect::kAfterInterp_PremulType == ge.getPremulType()) {
                 fragBuilder->codeAppend("colorTemp.rgb *= colorTemp.a;");
             }
             fragBuilder->codeAppendf("%s = %s;", outputColor,
                                      (GrGLSLExpr4(inputColor) * GrGLSLExpr4("colorTemp")).c_str());
 
             break;
         }
 
@@ skipping 290 matching lines @@
             (*stops)[i] = stop;
             stop = i < outColors - 1 ? stop + random->nextUScalar1() * (1.f - stop) : 1.f;
         }
     }
     *tm = static_cast<SkShader::TileMode>(random->nextULessThan(SkShader::kTileModeCount));
 
     return outColors;
 }
 
 #endif