Add a GM that reproduces layout test failures with my new xfermode code.

tests/SkNxTest.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 "Sk4px.h"
 #include "SkNx.h"
 #include "SkRandom.h"
@@ skipping 115 matching lines @@
 DEF_TEST(SkNi, r) {
     test_Ni<2, uint16_t>(r);
     test_Ni<4, uint16_t>(r);
     test_Ni<8, uint16_t>(r);
 
     test_Ni<2, int>(r);
     test_Ni<4, int>(r);
     test_Ni<8, int>(r);
 }
 
-DEF_TEST(SkNi_min, r) {
+DEF_TEST(SkNi_min_lt, r) {
     // Exhaustively check the 8x8 bit space.
     for (int a = 0; a < (1<<8); a++) {
     for (int b = 0; b < (1<<8); b++) {
-        REPORTER_ASSERT(r, Sk16b::Min(Sk16b(a), Sk16b(b)).kth<0>() == SkTMin(a, b));
+        Sk16b aw(a), bw(b);
+        REPORTER_ASSERT(r, Sk16b::Min(aw, bw).kth<0>() == SkTMin(a, b));
+        REPORTER_ASSERT(r, !(aw < bw).kth<0>() == !(a < b));
     }}
 
     // Exhausting the 16x16 bit space is kind of slow, so only do that in release builds.
 #ifdef SK_DEBUG
     SkRandom rand;
     for (int i = 0; i < (1<<16); i++) {
         uint16_t a = rand.nextU() >> 16,
                  b = rand.nextU() >> 16;
         REPORTER_ASSERT(r, Sk8h::Min(Sk8h(a), Sk8h(b)).kth<0>() == SkTMin(a, b));
     }
@@ skipping 32 matching lines @@
 
         // We're a bit more flexible on this method: correct for 0 or 255, otherwise off by <=1.
         int fast = av.approxMulDiv255(bv).kth<0>();
         REPORTER_ASSERT(r, fast-exact >= -1 && fast-exact <= 1);
         if (a == 0 || a == 255 || b == 0 || b == 255) {
             REPORTER_ASSERT(r, fast == exact);
         }
     }
     }
 }