R=fbarchard@google.com
BUG=libyuv:634

Performance gains as below,

YUY2ToI422, YUY2ToI420 :-

YUY2ToYRow_MSA          : ~10x
YUY2ToUVRow_MSA         : ~11x
YUY2ToUV422Row_MSA      : ~9x
YUY2ToYRow_Any_MSA      : ~6x
YUY2ToUVRow_Any_MSA     : ~5x
YUY2ToUV422Row_Any_MSA  : ~4x

UYVYToI422, UYVYToI420 :-

UYVYToYRow_MSA          : ~10x
UYVYToUVRow_MSA         : ~11x
UYVYToUV422Row_MSA      : ~9x
UYVYToYRow_Any_MSA      : ~6x
UYVYToUVRow_Any_MSA     : ~5x
UYVYToUV422Row_Any_MSA  : ~4x

Review URL: https://codereview.chromium.org/2397693002 .