Use vbroadcastf128 to copy m128 to ymm duplicating the value to high and low 128 bits.  Allows shared variables.
BUG=none
TEST=avx2 unittests still pass.
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2324004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@803 16f28f9a-4ce2-e073-06de-1de4eb20be90