offset to second group of pixels was off by 16.
should have been 32, not 16.
requires avx2 hardware and wide image for test.

R=harryjin@google.com
TBR=harryjin@google.com
BUG=libyuv:492,libyuv:501

Review URL: https://codereview.chromium.org/1395603002 .