Refactor to put SkXfermode_opts inside SK_OPTS_NS.

src/opts/SkOpts_sse2.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 "SkOpts.h"
 
 #define SK_OPTS_NS sse2
 #include "SkBlitMask_opts.h"
 #include "SkBlurImageFilter_opts.h"
 #include "SkMorphologyImageFilter_opts.h"
 #include "SkUtils_opts.h"
 #include "SkXfermode_opts.h"
 
 namespace SkOpts {
     void Init_sse2() {
         memset16        = sse2::memset16;
         memset32        = sse2::memset32;
-        create_xfermode = SkCreate4pxXfermode;
+        create_xfermode = sse2::create_xfermode;
 
         box_blur_xx = sse2::box_blur_xx;
         box_blur_xy = sse2::box_blur_xy;
         box_blur_yx = sse2::box_blur_yx;
 
         dilate_x = sse2::dilate_x;
         dilate_y = sse2::dilate_y;
          erode_x = sse2::erode_x;
          erode_y = sse2::erode_y;
 
         blit_mask_d32_a8 = sse2::blit_mask_d32_a8;
     }
 }