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

Performance Gain (vs C auto-vectorized)
NV12ToARGBRow_MSA       - ~1.5x
NV12ToARGBRow_Any_MSA   - ~1.4x
NV12ToRGB565Row_MSA     - ~1.4x
NV12ToRGB565Row_Any_MSA - ~1.4x
NV21ToARGBRow_MSA       - ~1.5x
NV21ToARGBRow_Any_MSA   - ~1.5x
SobelRow_MSA            - ~4.3x
SobelRow_Any_MSA        - ~3.4x
SobelToPlaneRow_MSA     - ~8.0x
SobelToPlaneRow_Any_MSA - ~4.7x
SobelXYRow_MSA          - ~3.0x
SobelXYRow_Any_MSA      - ~2.5x

Performance Gain (vs C non-vectorized)
NV12ToARGBRow_MSA       - ~6.5x
NV12ToARGBRow_Any_MSA   - ~6.5x
NV12ToRGB565Row_MSA     - ~6.2x
NV12ToRGB565Row_Any_MSA - ~6.1x
NV21ToARGBRow_MSA       - ~6.5x
NV21ToARGBRow_Any_MSA   - ~6.5x
SobelRow_MSA            - ~14.5x
SobelRow_Any_MSA        - ~11.3x
SobelToPlaneRow_MSA     - ~34.2x
SobelToPlaneRow_Any_MSA - ~19.4x
SobelXYRow_MSA          - ~11.1x
SobelXYRow_Any_MSA      - ~9.1x

Review-Url: https://codereview.chromium.org/2636483002 .