[FFmpeg-devel] [aarch64] improve performance of ff_hscale_8_to_15_neon

[Sebastian Pop]

Hi,

This patch implements ff_hscale_8_to_15_neon with NEON fused multiply accumulate
and bumps the vectorization factor from 2 to 4. I have seen speedups up to 15%
on Graviton A1 instances based on A-72 cpus.

$ ffmpeg -nostats -f lavfi -i testsrc2=4k:d=2 -vf
bench=start,scale=1024x1024,bench=stop -f null -
before: t:0.040303 avg:0.040287 max:0.040371 min:0.039214
after:  t:0.037339 avg:0.037327 max:0.037550 min:0.036992

Tested with `make check` on aarch64-linux.
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