scalerow avx2 bug fix. was using ymm2 instead of ymm3.

R=harryjin@google.com
BUG=libyuv:462

Review URL: https://webrtc-codereview.appspot.com/56639004.

include/libyuv/scale_row.h

@@ -50,8 +50,7 @@ extern "C" {
 
 // The following are available on VS2012:
 #if !defined(LIBYUV_DISABLE_X86) && defined(VISUALC_HAS_AVX2)
-// Some AVX2 versions disabled. See libyuv bug 462.
-// #define HAS_SCALEADDROW_AVX2
+#define HAS_SCALEADDROW_AVX2
 #define HAS_SCALEROWDOWN2_AVX2
 #define HAS_SCALEROWDOWN4_AVX2
 #endif

source/scale_win.cc

@@ -838,14 +838,12 @@ void ScaleAddRow_AVX2(const uint8* src_ptr, uint16* dst_ptr, int src_width) {
   // sum rows
   xloop:
     vmovdqu     ymm3, [eax]       // read 32 bytes
-    vpermq      ymm3, ymm2, 0xd8  // unmutate for vpunpck
     lea         eax, [eax + 32]
-    vmovdqu     ymm0, [edx]       // read 32 words from destination
-    vmovdqu     ymm1, [edx + 32]
+    vpermq      ymm3, ymm3, 0xd8  // unmutate for vpunpck
     vpunpcklbw  ymm2, ymm3, ymm5
     vpunpckhbw  ymm3, ymm3, ymm5
-    vpaddusw    ymm0, ymm0, ymm2  // sum 16 words
-    vpaddusw    ymm1, ymm1, ymm3
+    vpaddusw    ymm0, ymm2, [edx] // sum 16 words
+    vpaddusw    ymm1, ymm3, [edx + 32]
     vmovdqu     [edx], ymm0       // write 32 words to destination
     vmovdqu     [edx + 32], ymm1
     lea         edx, [edx + 64]