avcodec/vp9/x86: add ff_vp9_loop_filter_[vh]_16_16_ssse3().

16795 decicycles in loop_filter_h_16_16_c, 16773104 runs, 4112 skips
17667 decicycles in loop_filter_v_16_16_c, 16773311 runs, 3905 skips

5259 decicycles in ff_vp9_loop_filter_h_16_16_ssse3, 8386752 runs, 1856 skips
3907 decicycles in ff_vp9_loop_filter_v_16_16_ssse3, 8387438 runs, 1170 skips

Overall decode time goes from:
  ./ffmpeg -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  8.20s user 0.02s system 99% cpu 8.225 total
to:
  ./ffmpeg -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  6.28s user 0.02s system 99% cpu 6.307 total

(46 to 60 fps)