arm64: mc: Apply tuning from w4/w8 case to w2 case in 16 bpc 8tap_hv
arm64: mc: Apply tuning from w4/w8 case to w2 case in 16 bpc 8tap_hv

Narrowing the intermediates from the horizontal pass is beneficial
(on most cores, but a small slowdown on A53) here as well. This
increases consistency in the code between the cases.

(The corresponding change in the upcoming arm32 version is beneficial
on all tested cores except for on A53 - it helps, on some cores a lot,
on A7, A8, A9, A72, A73 and only makes it marginally slower on A53.)

Before:                        Cortex A53     A72     A73
mc_8tap_regular_w2_hv_16bpc_neon:   457.7   301.0   317.1
After:
mc_8tap_regular_w2_hv_16bpc_neon:   472.0   276.0   284.3

xiph/rav1e,xiph/rav1e