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

16662 decicycles in loop_filter_h_16_16_c, 8387355 runs, 1253 skips
17510 decicycles in loop_filter_v_16_16_c, 8387516 runs, 1092 skips

4941 decicycles in ff_vp9_loop_filter_h_16_16_ssse3, 8387887 runs, 721 skips
3899 decicycles in ff_vp9_loop_filter_v_16_16_ssse3, 8387980 runs, 628 skips

Overall decode time goes from:
  ./ffmpeg -v 0 -nostats -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  8.10s user 0.02s system 99% cpu 8.126 total
to:
  ./ffmpeg -v 0 -nostats -threads 1 -i ~/samples/vp9/ped1080p.webm -f null -  6.15s user 0.04s system 99% cpu 6.199 total

(46 to 61 fps)

libavcodec/x86/Makefile

@@ -102,5 +102,6 @@ YASM-OBJS-$(CONFIG_VP6_DECODER)        += x86/vp6dsp.o
 YASM-OBJS-$(CONFIG_VP8_DECODER)        += x86/vp8dsp.o                  \
                                           x86/vp8dsp_loopfilter.o
 YASM-OBJS-$(CONFIG_VP9_DECODER)        += x86/vp9itxfm.o                \
+                                          x86/vp9lpf.o                  \
                                           x86/vp9mc.o
 YASM-OBJS-$(CONFIG_WEBP_DECODER)       += x86/vp8dsp.o

libavcodec/x86/vp9dsp_init.c

@@ -162,6 +162,9 @@ void ff_vp9_idct_idct_8x8_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *blo
 void ff_vp9_idct_idct_16x16_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *block, int eob);
 void ff_vp9_idct_idct_32x32_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *block, int eob);
 
+void ff_vp9_loop_filter_v_16_16_ssse3(uint8_t *dst, ptrdiff_t stride, int E, int I, int H);
+void ff_vp9_loop_filter_h_16_16_ssse3(uint8_t *dst, ptrdiff_t stride, int E, int I, int H);
+
 #endif /* HAVE_YASM */
 
 av_cold void ff_vp9dsp_init_x86(VP9DSPContext *dsp)
@@ -223,6 +226,8 @@ av_cold void ff_vp9dsp_init_x86(VP9DSPContext *dsp)
             dsp->itxfm_add[TX_32X32][ADST_DCT] =
             dsp->itxfm_add[TX_32X32][DCT_ADST] =
             dsp->itxfm_add[TX_32X32][DCT_DCT] = ff_vp9_idct_idct_32x32_add_ssse3;
+            dsp->loop_filter_16[0] = ff_vp9_loop_filter_h_16_16_ssse3;
+            dsp->loop_filter_16[1] = ff_vp9_loop_filter_v_16_16_ssse3;
         }
     }