Use ld2 to load even and odd pixels into different registers
and hadd to half add them to each other.

Previously used paired and shift.

TBR=kjellander@chromium.org
BUG=libyuv:723
TEST=ScaleDownBy2_Linear

Change-Id: I3ec72bcf7d4c746837217496c301eb4e4ad963cf
Reviewed-on: https://chromium-review.googlesource.com/644113Reviewed-by: Cheng Wang <wangcheng@google.com>