Use SIMD instructions when counting a single byte.
Inspired from runtime IndexByte implementation.

Benchmark results of bytes, where 1 byte in every 8 is the one we are looking:

name               old time/op   new time/op    delta
CountSingle/10-8    96.1ns ± 1%    38.8ns ± 0%    -59.64%  (p=0.000 n=9+7)
CountSingle/32-8     172ns ± 2%      36ns ± 1%    -79.27%  (p=0.000 n=10+10)
CountSingle/4K-8    18.2µs ± 1%     0.9µs ± 0%    -95.17%  (p=0.000 n=9+10)
CountSingle/4M-8    18.4ms ± 0%     0.9ms ± 0%    -95.00%  (p=0.000 n=10+9)
CountSingle/64M-8    284ms ± 4%      19ms ± 0%    -93.40%  (p=0.000 n=10+10)

name               old speed     new speed      delta
CountSingle/10-8   104MB/s ± 1%   258MB/s ± 0%   +147.99%  (p=0.000 n=9+10)
CountSingle/32-8   185MB/s ± 1%   897MB/s ± 1%   +385.33%  (p=0.000 n=9+10)
CountSingle/4K-8   225MB/s ± 1%  4658MB/s ± 0%  +1967.40%  (p=0.000 n=9+10)
CountSingle/4M-8   228MB/s ± 0%  4555MB/s ± 0%  +1901.71%  (p=0.000 n=10+9)
CountSingle/64M-8  236MB/s ± 4%  3575MB/s ± 0%  +1414.69%  (p=0.000 n=10+10)

Change-Id: Ifccb51b3c8658c49773fe05147c3cf3aead361e5
Reviewed-on: https://go-review.googlesource.com/71111Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>