x86/vp9lpf: add ff_vp9_loop_filter_[vh]_88_16_{ssse3,avx}.

9680 decicycles in loop_filter_v_88_16_c, 4193765 runs, 539 skips
9233 decicycles in loop_filter_h_88_16_c, 4193751 runs, 553 skips

1932 decicycles in ff_vp9_loop_filter_v_88_16_ssse3, 4194162 runs, 142 skips
3058 decicycles in ff_vp9_loop_filter_h_88_16_ssse3, 4193877 runs, 427 skips

5.994 → 5.478 overall decode time on ped1080p.webm (-threads 1)

Adding SSE2 support should be relatively trivial (just a matter of
changing the pshufb [mask_mix] with something else), patch welcome.