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

Performance gains :-

I422ToYUY2Row_MSA     - ~12x
I422ToYUY2Row_Any_MSA - ~7x

I422ToUYVYRow_MSA     - ~12x
I422ToUYVYRow_Any_MSA - ~7x

Review URL: https://codereview.chromium.org/2378753004 .