avcodec/vp9: add ff_vp9_idct_idct_{4x4,8x8}_ssse3().

1789 decicycles in idct_idct_4x4_add_c, 262136 runs, 8 skips
1839 decicycles in idct_idct_4x4_add_c, 524270 runs, 18 skips
1864 decicycles in idct_idct_4x4_add_c, 1048548 runs, 28 skips

529 decicycles in ff_vp9_idct_idct_4x4_add_ssse3, 262138 runs, 6 skips
516 decicycles in ff_vp9_idct_idct_4x4_add_ssse3, 524282 runs, 6 skips
474 decicycles in ff_vp9_idct_idct_4x4_add_ssse3, 1048565 runs, 11 skips

(~3.9x faster)

7726 decicycles in idct_idct_8x8_add_c, 1048433 runs, 143 skips
7732 decicycles in idct_idct_8x8_add_c, 2096882 runs, 270 skips
7731 decicycles in idct_idct_8x8_add_c, 4193772 runs, 532 skips

1145 decicycles in ff_vp9_idct_idct_8x8_add_ssse3, 1048549 runs, 27 skips
1137 decicycles in ff_vp9_idct_idct_8x8_add_ssse3, 2097097 runs, 55 skips
1086 decicycles in ff_vp9_idct_idct_8x8_add_ssse3, 4194188 runs, 116 skips

(~7.1x faster)

Overall decode time before commit:
  16.48s user 0.03s system 99% cpu 16.526 total
  16.54s user 0.01s system 99% cpu 16.566 total
  16.46s user 0.03s system 99% cpu 16.511 total

Overall decode time after commit:
  16.34s user 0.02s system 99% cpu 16.378 total
  16.28s user 0.02s system 99% cpu 16.315 total
  16.32s user 0.03s system 99% cpu 16.366 total

Tested on i7 920 with 40s 1080p footage.

libavcodec/x86/vp9dsp_init.c

@@ -150,6 +150,9 @@ filters_8tap_1d_fn3(avg)
 #undef filters_8tap_1d_fn3
 #undef filter_8tap_1d_fn
 
+void ff_vp9_idct_idct_4x4_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *block, int eob);
+void ff_vp9_idct_idct_8x8_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *block, int eob);
+
 #endif /* HAVE_YASM */
 
 av_cold void ff_vp9dsp_init_x86(VP9DSPContext *dsp)
@@ -203,6 +206,9 @@ av_cold void ff_vp9dsp_init_x86(VP9DSPContext *dsp)
     if (cpu_flags & AV_CPU_FLAG_SSSE3) {
         init_subpel3(0, put, ssse3);
         init_subpel3(1, avg, ssse3);
+        dsp->itxfm_add[TX_4X4][DCT_DCT] = ff_vp9_idct_idct_4x4_add_ssse3;
+        if (ARCH_X86_64)
+            dsp->itxfm_add[TX_8X8][DCT_DCT] = ff_vp9_idct_idct_8x8_add_ssse3;
     }
 
 #undef init_fpel