arm64: ipred: NEON implementation of smooth prediction
arm64: ipred: NEON implementation of smooth prediction

Relative speedups over the C code:
                               Cortex A53    A72    A73
intra_pred_smooth_h_w4_8bpc_neon:    8.02   4.53   7.09
intra_pred_smooth_h_w8_8bpc_neon:   16.59   5.91   9.32
intra_pred_smooth_h_w16_8bpc_neon:  18.80   5.54  10.10
intra_pred_smooth_h_w32_8bpc_neon:   5.07   4.43   4.60
intra_pred_smooth_h_w64_8bpc_neon:   5.03   4.26   4.34
intra_pred_smooth_v_w4_8bpc_neon:    9.11   5.51   7.75
intra_pred_smooth_v_w8_8bpc_neon:   17.07   6.86  10.55
intra_pred_smooth_v_w16_8bpc_neon:  17.98   6.38  11.52
intra_pred_smooth_v_w32_8bpc_neon:  11.69   5.66   8.09
intra_pred_smooth_v_w64_8bpc_neon:   8.44   4.34   5.72
intra_pred_smooth_w4_8bpc_neon:      9.81   4.85   6.93
intra_pred_smooth_w8_8bpc_neon:     16.05   5.60   9.26
intra_pred_smooth_w16_8bpc_neon:    14.01   5.02   8.96
intra_pred_smooth_w32_8bpc_neon:     9.29   5.02   7.25
intra_pred_smooth_w64_8bpc_neon:     6.53   3.94   5.26

xiph/rav1e,xiph/rav1e