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

Performance gains : (Auto-vectorized C vs MSA SIMD)

ARGB4444ToYRow_MSA        : ~3.0x
ARGB4444ToUVRow_MSA       : ~1.8x
ARGB4444ToARGBRow_MSA     : ~3.4x

ARGB4444ToYRow_Any_MSA    : ~2.8x
ARGB4444ToUVRow_Any_MSA   : ~1.7x
ARGB4444ToARGBRow_Any_MSA : ~3.2x

Review URL: https://codereview.chromium.org/2421843002 .