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

Performance Gain (vs C vectorized)
ARGBMultiplyRow_MSA       - 1.4x
ARGBAddRow_MSA            - 8.6x
ARGBSubtractRow_MSA       - 8.6x

ARGBMultiplyRow_Any_MSA   - 1.35x
ARGBAddRow_Any_MSA        - 7.3x
ARGBSubtractRow_Any_MSA   - 7.2x

Performance Gain (vs C non-vectorized)
ARGBMultiplyRow_MSA       - 4.4x
ARGBAddRow_MSA            - 27x
ARGBSubtractRow_MSA       - 22x

ARGBMultiplyRow_Any_MSA   - 3.5x
ARGBAddRow_Any_MSA        - 23x
ARGBSubtractRow_Any_MSA   - 18x

Review URL: https://codereview.chromium.org/2529983002 .