aarch64: Improve scheduling in sad_x3/sad_x4

               Cortex A53    A72    A73
8 bpc:
Before:
sad_x3_4x4_neon:      580    303    204
sad_x3_4x8_neon:     1065    516    323
sad_x3_8x4_neon:      668    262    282
sad_x3_8x8_neon:     1238    454    471
sad_x3_8x16_neon:    2378    842    847
sad_x3_16x8_neon:    2136    738    776
sad_x3_16x16_neon:   4162   1378   1463
After:
sad_x3_4x4_neon:      477    298    206
sad_x3_4x8_neon:      842    515    327
sad_x3_8x4_neon:      603    260    279
sad_x3_8x8_neon:     1110    451    464
sad_x3_8x16_neon:    2125    841    843
sad_x3_16x8_neon:    2124    730    766
sad_x3_16x16_neon:   4145   1370   1434

10 bpc:
Before:
sad_x3_4x4_neon:      632    247    254
sad_x3_4x8_neon:     1162    419    443
sad_x3_8x4_neon:      890    358    416
sad_x3_8x8_neon:     1670    632    759
sad_x3_8x16_neon:    3230   1179   1458
sad_x3_16x8_neon:    3070   1209   1403
sad_x3_16x16_neon:   6030   2333   2699

After:
sad_x3_4x4_neon:      522    253    255
sad_x3_4x8_neon:      932    443    431
sad_x3_8x4_neon:      880    354    406
sad_x3_8x8_neon:     1660    626    736
sad_x3_8x16_neon:    3220   1170   1397
sad_x3_16x8_neon:    3060   1184   1362
sad_x3_16x16_neon:   6020   2272   2579

Thus, this is around a 20-25% speedup on Cortex A53 for the small sizes (much smaller difference for bigger sizes though), while it doesn't make much of a difference at all (mostly within measurement noise) for the out-of-order cores (A72 and A73).