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

Performance Gain (vs C vectorized)
ARGBAttenuateRow_MSA          - ~1.1x
ARGBAttenuateRow_Any_MSA      - ~1.1x
ARGBToRGB565DitherRow_MSA     - ~6.4x
ARGBToRGB565DitherRow_Any_MSA - ~6.2x
ARGBShuffleRow_MSA            - ~5.1x
ARGBShuffleRow_Any_MSA        - ~1.9x
ARGBShadeRow_MSA              - ~1.1x
ARGBGrayRow_MSA               - ~2.6x
ARGBSepiaRow_MSA              - ~11.6x

Performance Gain (vs C non-vectorized)
ARGBAttenuateRow_MSA          - ~2.46x
ARGBAttenuateRow_Any_MSA      - ~2.45x
ARGBToRGB565DitherRow_MSA     - ~9.4x
ARGBToRGB565DitherRow_Any_MSA - ~12.5x
ARGBShuffleRow_MSA            - ~5.2x
ARGBShuffleRow_Any_MSA        - ~1.9x
ARGBShadeRow_MSA              - ~4.3x
ARGBGrayRow_MSA               - ~10.5x
ARGBSepiaRow_MSA              - ~12.2x

Review-Url: https://codereview.chromium.org/2559693002 .