Reduce alignment for loops from 16 bytes to 4 bytes.  Reduces outer loop overhead without hurting innerloop time.
BUG=none
TESTED=try bots
R=fbarchard@chromium.org, mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4659004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@880 16f28f9a-4ce2-e073-06de-1de4eb20be90