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 .