Simple single-byte loop count for now, to be further improved in future
CLs.

Benchmark on linux/arm:

name               old time/op    new time/op     delta
CountSingle/10-4      122ns ± 0%       87ns ± 1%  -28.41%  (p=0.000 n=7+10)
CountSingle/32-4      242ns ± 0%      174ns ± 1%  -28.25%  (p=0.000 n=10+10)
CountSingle/4K-4     24.2µs ± 1%     15.6µs ± 1%  -35.42%  (p=0.000 n=10+10)
CountSingle/4M-4     29.6ms ± 1%     21.3ms ± 1%  -28.09%  (p=0.000 n=10+9)
CountSingle/64M-4     562ms ± 0%      414ms ± 1%  -26.23%  (p=0.000 n=8+10)

name               old speed      new speed       delta
CountSingle/10-4   81.7MB/s ± 1%  114.5MB/s ± 1%  +40.07%  (p=0.000 n=10+10)
CountSingle/32-4    132MB/s ± 0%    184MB/s ± 1%  +39.39%  (p=0.000 n=10+9)
CountSingle/4K-4    170MB/s ± 1%    263MB/s ± 1%  +54.86%  (p=0.000 n=10+10)
CountSingle/4M-4    142MB/s ± 1%    197MB/s ± 1%  +39.07%  (p=0.000 n=10+9)
CountSingle/64M-4   119MB/s ± 0%    162MB/s ± 1%  +35.55%  (p=0.000 n=8+10)

Updates #29001

Change-Id: I42a268215a62044286ec32b548d8e4b86b9570ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/168319
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>