wc -l AVX code 10%+10% speedup

[Evgeny Nizhibitsky]

Dear GNU coreutils maintainers,

It seems that I found a way to both speed-up (~10%) and simplify (13
insertions, 43 deletions) the wc -l avx code while playing with it, at
least on several million to 1 billion row files I tested with my cpu.

It mostly involves using _mm256_movemask_epi8 and __builtin_popcount
instead of the two accumulators handling that allowed me to increase the
buffer size.

I also have a further ~10% improvement in code by using 2 separate threads
instead of 1 to mitigate the usr time overhead, although it’s naturally
more complicated.

Whom should I discuss this potential contribution with?

Best wishes,
Evgeny