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

Performance Gain (vs C vectorized)
I422AlphaToARGBRow_MSA      : ~1.4x
I422AlphaToARGBRow_Any_MSA  : ~1.4x
I422ToRGB24Row_MSA          : ~4.8x
I422ToRGB24Row_Any_MSA      : ~4.8x

Performance Gain (vs C non-vectorized)
I422AlphaToARGBRow_MSA      : ~7.0x
I422AlphaToARGBRow_Any_MSA  : ~7.0x
I422ToRGB24Row_MSA          : ~7.9x
I422ToRGB24Row_Any_MSA      : ~7.7x

Review URL: https://codereview.chromium.org/2454433003 .