Add SSSE3 implementation for ipred_smooth, ipred_smooth_v and ipred_smooth_h

intra_pred_smooth_h_w4_8bpc_c: 460.6
intra_pred_smooth_h_w4_8bpc_ssse3: 83.1
intra_pred_smooth_h_w8_8bpc_c: 1286.9
intra_pred_smooth_h_w8_8bpc_ssse3: 172.0
intra_pred_smooth_h_w16_8bpc_c: 3804.8
intra_pred_smooth_h_w16_8bpc_ssse3: 460.3
intra_pred_smooth_h_w32_8bpc_c: 8505.0
intra_pred_smooth_h_w32_8bpc_ssse3: 1176.9
intra_pred_smooth_h_w64_8bpc_c: 22236.9
intra_pred_smooth_h_w64_8bpc_ssse3: 2810.8

intra_pred_smooth_v_w4_8bpc_c: 433.2
intra_pred_smooth_v_w4_8bpc_ssse3: 75.6
intra_pred_smooth_v_w8_8bpc_c: 1279.4
intra_pred_smooth_v_w8_8bpc_ssse3: 134.2
intra_pred_smooth_v_w16_8bpc_c: 4060.8
intra_pred_smooth_v_w16_8bpc_ssse3: 333.0
intra_pred_smooth_v_w32_8bpc_c: 9758.9
intra_pred_smooth_v_w32_8bpc_ssse3: 1423.2
intra_pred_smooth_v_w64_8bpc_c: 26571.8
intra_pred_smooth_v_w64_8bpc_ssse3: 3534.1

intra_pred_smooth_w4_8bpc_c: 1138.4
intra_pred_smooth_w4_8bpc_ssse3: 113.8
intra_pred_smooth_w8_8bpc_c: 3378.8
intra_pred_smooth_w8_8bpc_ssse3: 257.3
intra_pred_smooth_w16_8bpc_c: 10660.1
intra_pred_smooth_w16_8bpc_ssse3: 711.5
intra_pred_smooth_w32_8bpc_c: 20899.8
intra_pred_smooth_w32_8bpc_ssse3: 2275.0
intra_pred_smooth_w64_8bpc_c: 43132.2
intra_pred_smooth_w64_8bpc_ssse3: 5918.2