Scale down by 2 AVX2 port.  Processes twice as many pixels as SSE2 and takes advantage of 3 argument instructions to reduce register usage and number of instructions.
BUG=314
TESTED=libyuvTest.ScaleDownBy2_Box
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/42959004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1347 16f28f9a-4ce2-e073-06de-1de4eb20be90