urhadd is a rounded average.  Linear filter wants to average
horizontally, so use ld2 to separate even and odd pixels.

TBR=jkellander@chromium.org
BUG=None
TEST=LibYUVScaleTest.*ScaleDownBy2*

Change-Id: Id667288a030e72ce8e1c1d6719b69c555c0db063
Reviewed-on: https://chromium-review.googlesource.com/642448
Commit-Queue: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>