Fix aligned version of SSSE3 alpha blend and re-enable SSE2 alpha blend for performance evaluation on low end CPUs where pshufb is slow.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/728009

git-svn-id: http://libyuv.googlecode.com/svn/trunk@322 16f28f9a-4ce2-e073-06de-1de4eb20be90