Add vertical loopfilter SIMD.

lpf_v_sb128uv_w4_8bpc_c: 568.2
lpf_v_sb128uv_w4_8bpc_avx2: 60.8
lpf_v_sb128uv_w6_8bpc_c: 744.2
lpf_v_sb128uv_w6_8bpc_avx2: 135.7
lpf_v_sb128y_w4_8bpc_c: 1046.4
lpf_v_sb128y_w4_8bpc_avx2: 118.0
lpf_v_sb128y_w8_8bpc_c: 1441.8
lpf_v_sb128y_w8_8bpc_avx2: 292.9
lpf_v_sb128y_w16_8bpc_c: 1778.7
lpf_v_sb128y_w16_8bpc_avx2: 577.5

    $ time tools/dav1d -i ~/Downloads/Chimera-AV1-8bit-1920x1080-6736kbps.ivf --limit=1000 -o /dev/null --muxer=yuv
    before:
    real        0m30.496s
    user        0m30.230s
    after:
    real        0m29.818s
    user        0m29.556s