Add SSSE3 implementation for dav1d_ipred_h

Cycle times:

intra_pred_h_w4_8bpc_c: 146.6
intra_pred_h_w4_8bpc_ssse3: 30.6
intra_pred_h_w8_8bpc_c: 236.3
intra_pred_h_w8_8bpc_ssse3: 42.2
intra_pred_h_w16_8bpc_c: 446.6
intra_pred_h_w16_8bpc_ssse3: 55.8
intra_pred_h_w32_8bpc_c: 688.2
intra_pred_h_w32_8bpc_ssse3: 85.9
intra_pred_h_w64_8bpc_c: 634.2
intra_pred_h_w64_8bpc_ssse3: 169.2