Move vzeroupper to row functions to simplify caller and allow mix of avx2 and sse2.  Impact reduced by row coalescing.
BUG=none
TEST=all tests pass with sde
Review URL: https://webrtc-codereview.appspot.com/1269009

git-svn-id: http://libyuv.googlecode.com/svn/trunk@641 16f28f9a-4ce2-e073-06de-1de4eb20be90