Janne Grunau authored Aug 17, 2015 and Henrik Gramner committed Oct 11, 2015

Make them at least as fast as the compiled C version (tested on
cortex-a53 vs. gcc 4.9.2).

                        C     NEON (before)   NEON (after)
intra_predict_4x4_dc:   260   335             260
intra_predict_4x4_dct:  210   265             200
intra_predict_8x8c_dc:  497   548             493
intra_predict_8x8c_v:   232   309             179 (arm64)
intra_predict_8x16c_dc: 795   830             790