Also make sure that the w4 case can exit after processing 12 pixels,
where it is convenient.

This gives a small slowdown for in-order cores like A7, A8, A53, but
acutally seems to give a small speedup for out-of-order cores like
A9, A72 and A73.

AArch64:
Before:                      Cortex A53     A72     A73
mc_8tap_regular_w8_v_8bpc_neon:   223.8   247.3   228.5
After:
mc_8tap_regular_w8_v_8bpc_neon:   232.5   243.9   223.4

AArch32:
Before:                       Cortex A7      A8      A9     A53     A72     A73
mc_8tap_regular_w8_v_8bpc_neon:   550.2   470.7   520.5   257.0   256.4   248.2
After:
mc_8tap_regular_w8_v_8bpc_neon:   554.3   474.2   511.6   267.5   252.6   246.8