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

Performance Gain (vs C vectorized)
ScaleARGBRowDown2_MSA           - ~2.6x
ScaleARGBRowDown2Linear_MSA     - ~7.9x
ScaleARGBRowDown2Box_MSA        - ~3.7x
ScaleARGBRowDownEven_MSA        - ~1.2x
ScaleARGBRowDownEvenBox_MSA     - ~3.5x

ScaleARGBRowDown2_Any_MSA       - ~2.6x
ScaleARGBRowDown2Linear_Any_MSA - ~7.9x
ScaleARGBRowDown2Box_Any_MSA    - ~3.6x
ScaleARGBRowDownEven_Any_MSA    - ~1.2x
ScaleARGBRowDownEvenBox_Any_MSA - ~3.5x

Performance Gain (vs C non-vectorized)
ScaleARGBRowDown2_MSA           - 2.6x
ScaleARGBRowDown2Linear_MSA     - 13.5x
ScaleARGBRowDown2Box_MSA        - 5.8x
ScaleARGBRowDownEven_MSA        - 1.2x
ScaleARGBRowDownEvenBox_MSA     - 3.7x

ScaleARGBRowDown2_Any_MSA       - 2.6x
ScaleARGBRowDown2Linear_Any_MSA - 13.5x
ScaleARGBRowDown2Box_Any_MSA    - 5.3x
ScaleARGBRowDownEven_Any_MSA    - 1.2x
ScaleARGBRowDownEvenBox_Any_MSA - 3.7x

Review URL: https://codereview.chromium.org/2527983002 .