Neon version of vp9_sub_pixel_variance8x8(),

vp9_variance8x8(), and vp9_get8x8var().

On a Nexus 7, vpxenc (in realtime mode, speed -12)
reported a performance improvement of ~1.2%.

Change-Id: I8a66ac2a0f550b407caa27816833bdc563395102
3 files changed