Port SSE2 version to AVX2.
BUG=libyuv:572
TEST=/usr/local/google/home/fbarchard/intelsde/sde -skx -- out/Release/libyuv_unittest --gtest_filter=*Extract*
R=wangcheng@google.com, magjed@chromium.org

Review URL: https://codereview.chromium.org/2420553002 .