Change elif to endif and if to allow AVX2 as well as SSE2 in future changes instead of one or the other.
BUG=none
TESTED=try bots
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/30719004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1122 16f28f9a-4ce2-e073-06de-1de4eb20be90