YUV to RGB for x64 use registers instead of memory.

On Arm the YVU to RGB conversions move constants into registers.
This change does the same for 64 bit intel builds where additional
registers are available.
The AVX2 saves 3 instructions by because the 2nd argument needs to be a register, so a vmovdqu was avoided.

x64 builds using memory:
AVX2  I420ToARGB_Opt (3059 ms)
SSSE3 I420ToARGB_Opt (3959 ms)

Now using registers
AVX2  I420ToARGB_Opt (2906 ms)
SSSE3 I420ToARGB_Opt (3928 ms)

TBR=harryjin@google.com
BUG=libyuv:520

Review URL: https://codereview.chromium.org/1407353010 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1531
+Version: 1532
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1531
+#define LIBYUV_VERSION 1532
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT