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

Performance Gain (vs C vectorized)
I422ToRGB565Row_MSA             : ~1.5x
I422ToRGB565Row_Any_MSA         : ~1.5x
I422ToARGB4444Row_MSA           : ~1.4x
I422ToARGB4444Row_Any_MSA       : ~1.4x
I422ToARGB1555Row_MSA           : ~1.4x
I422ToARGB1555Row_Any_MSA       : ~1.4x

Performance Gain (vs C non-vectorized)
I422ToRGB565Row_MSA             : ~6.8x
I422ToRGB565Row_Any_MSA         : ~6.8x
I422ToARGB4444Row_MSA           : ~6.6x
I422ToARGB4444Row_Any_MSA       : ~6.6x
I422ToARGB1555Row_MSA           : ~6.6x
I422ToARGB1555Row_Any_MSA       : ~6.6x

Review URL: https://codereview.chromium.org/2445343007 .