add ScaleMaxSamples_NEON function with max
done on original values.

TBR=kjellander@chromium.org
BUG=libyuv:717
TEST=LibYUVPlanarTest.TestScaleMaxSamples_Opt

Change-Id: Id99338860782b10ffd24f66242eb42014c2e229e
Reviewed-on: https://chromium-review.googlesource.com/614685Reviewed-by: Frank Barchard <fbarchard@google.com>
Reviewed-by: Cheng Wang <wangcheng@google.com>