arm64   8     TestARGBExtractAlpha (10019 ms) <-original 64 bit code
arm64   8 x2  TestARGBExtractAlpha (7639 ms)
arm64   16    TestARGBExtractAlpha (7369 ms) <- new 64 bit code
thumb32 8     TestARGBExtractAlpha (9505 ms) <- original 32 bit code
thumb32 8 x2  TestARGBExtractAlpha (7400 ms)
thumb32 8 x2i TestARGBExtractAlpha (7266 ms) <- new 32 bit code
arm32   8     TestARGBExtractAlpha (10002 ms)

BUG=libyuv:572
TESTED=local test on nexus 9
R=harryjin@google.com, wangcheng@google.com

Review URL: https://codereview.chromium.org/2035573002 .