ARGBToI400 and ARGBToI411 using AVX2. YUY2ToI420 and UVYVToI420 use AVX2. CopyPlane use rep movsb for AVX2.   CopyPlane2 use rep movsb for AVX2 and CopyPlane if strides match AVX2, which will do a single rep movsb for entire image if stride == width.  MergeUV for I420ToNV12.
BUG=181
TESTED=unittests pass
Review URL: https://webrtc-codereview.appspot.com/1103007

git-svn-id: http://libyuv.googlecode.com/svn/trunk@569 16f28f9a-4ce2-e073-06de-1de4eb20be90