Optimize I422ToRGBA for AVX2 by hoisting ymm5 initialization and using different register for output of unpack.
BUG=269
TESTED=intelsde on I422ToABGR
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/29889004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1137 16f28f9a-4ce2-e073-06de-1de4eb20be90