scale to half size optimization for avx2 - use pmaddubsw instruction to horizontally add bytes, then pavgw to round and divide by 2.
BUG=314
TESTED=libyuvTest.ScaleDownBy2*
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/45909004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1352 16f28f9a-4ce2-e073-06de-1de4eb20be90