vp9/x86: idct_add_16x16_ssse3.

Currently only dc-only and full 16x16. Other subforms will follow in the
near future. Total decoding time of ped1080p.webm goes from 9.7 to 9.3
seconds. DC-only goes from 957 -> 131 cycles, and the full IDCT goes
from ~4050 to ~745 cycles.

libavcodec/x86/vp9dsp_init.c

@@ -153,6 +153,7 @@ filters_8tap_1d_fn3(avg)
 
 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);
+void ff_vp9_idct_idct_16x16_add_ssse3(uint8_t *dst, ptrdiff_t stride, int16_t *block, int eob);
 
 #endif /* HAVE_YASM */
 
@@ -208,8 +209,10 @@ av_cold void ff_vp9dsp_init_x86(VP9DSPContext *dsp)
         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)
+        if (ARCH_X86_64) {
             dsp->itxfm_add[TX_8X8][DCT_DCT] = ff_vp9_idct_idct_8x8_add_ssse3;
+            dsp->itxfm_add[TX_16X16][DCT_DCT] = ff_vp9_idct_idct_16x16_add_ssse3;
+        }
     }
 
 #undef init_fpel