Martin Storsjö authored Jan 12, 2022 and Ronald S. Bultje committed Jan 13, 2022

For 8tap, unroll the vertical filters slightly less (by 4 instead of
8 elements) and add a special case trailer that handles only 2 elements
(for 2x6 and 4x6). By unrolling less, performance on in-order cores is
somewhat impacted.

Before:                      Cortex A53     A72     A73
mc_8tap_regular_w2_v_8bpc_neon:   146.5   141.3   145.6
mc_8tap_regular_w4_v_8bpc_neon:   175.2   180.3   162.4
After:
mc_8tap_regular_w2_v_8bpc_neon:   175.7   142.7   150.5
mc_8tap_regular_w4_v_8bpc_neon:   183.3   176.0   154.6