AVX2 version can process 16 pixels at a time for improved memory bandwidth and fewer instructions.

unittests improved to test unaligned memory, and test exactness when alpha is 0 or 255.

R=dhrosa@google.com, harryjin@google.com
BUG=libyuv:527

Review URL: https://codereview.chromium.org/1505433002 .