C     I420AlphaToARGB_Opt (5169 ms)
SSSE3 I420AlphaToARGB_Opt (432 ms)
AVX2  I420AlphaToARGB_Opt (358 ms)

and with premultiplication as 2 step process:
I420AlphaToARGB_Premult (7029 ms)
I420AlphaToARGB_Premult (757 ms)
I420AlphaToARGB_Premult (508 ms)

R=harryjin@google.com
BUG=libyuv:496,libyuv:473

Review URL: https://codereview.chromium.org/1372653003 .