internal/bytealg: improve PPC64 equal

internal/bytealg: improve PPC64 equal

Rewrite the vector loop to process 64B per iteration,
this greatly improves performance on POWER9/POWER10 for
large sizes.

Likewise, use a similar tricks for sizes >= 8 and <= 64.

And, rewrite small comparisons, it's a little slower for 1 byte,
but constant time for 1-7 bytes. Thus, it is increasingly faster
for 2-7B.

Benchmarks results below are from P8/P9 ppc64le (in that order),
several additional testcases have been added to test interesting
sizes. Likewise, the old variant was padded to the same code
size of the new variant to minimize layout related noise:

POWER8/ppc64le/linux:
name       old speed      new speed       delta
Equal/1     110MB/s ± 0%    106MB/s ± 0%    -3.26%
Equal/2     202MB/s ± 0%    203MB/s ± 0%    +0.18%
Equal/3     280MB/s ± 0%    319MB/s ± 0%   +13.89%
Equal/4     350MB/s ± 0%    414MB/s ± 0%   +18.27%
Equal/5     412MB/s ± 0%    533MB/s ± 0%   +29.19%
Equal/6     462MB/s ± 0%    620MB/s ± 0%   +34.11%
Equal/7     507MB/s ± 0%    745MB/s ± 0%   +47.02%
Equal/8     913MB/s ± 0%    994MB/s ± 0%    +8.84%
Equal/9     909MB/s ± 0%   1117MB/s ± 0%   +22.85%
Equal/10    937MB/s ± 0%   1242MB/s ± 0%   +32.59%
Equal/11    962MB/s ± 0%   1370MB/s ± 0%   +42.37%
Equal/12    989MB/s ± 0%   1490MB/s ± 0%   +50.60%
Equal/13   1.01GB/s ± 0%   1.61GB/s ± 0%   +60.27%
Equal/14   1.02GB/s ± 0%   1.74GB/s ± 0%   +71.22%
Equal/15   1.03GB/s ± 0%   1.86GB/s ± 0%   +81.45%
Equal/16   1.60GB/s ± 0%   1.99GB/s ± 0%   +24.21%
Equal/17   1.54GB/s ± 0%   2.04GB/s ± 0%   +32.28%
Equal/20   1.48GB/s ± 0%   2.40GB/s ± 0%   +62.64%
Equal/32   3.58GB/s ± 0%   3.84GB/s ± 0%    +7.18%
Equal/63   3.74GB/s ± 0%   7.17GB/s ± 0%   +91.79%
Equal/64   6.35GB/s ± 0%   7.29GB/s ± 0%   +14.75%
Equal/65   5.85GB/s ± 0%   7.00GB/s ± 0%   +19.66%
Equal/127  6.74GB/s ± 0%  13.74GB/s ± 0%  +103.77%
Equal/128  10.6GB/s ± 0%   12.9GB/s ± 0%   +21.98%
Equal/129  9.66GB/s ± 0%  11.96GB/s ± 0%   +23.85%
Equal/191  9.12GB/s ± 0%  17.80GB/s ± 0%   +95.26%
Equal/192  13.4GB/s ± 0%   17.2GB/s ± 0%   +28.66%
Equal/4K   29.5GB/s ± 0%   37.3GB/s ± 0%   +26.39%
Equal/4M   22.6GB/s ± 0%   23.1GB/s ± 0%    +2.40%
Equal/64M  10.6GB/s ± 0%   11.2GB/s ± 0%    +5.83%

POWER9/ppc64le/linux:
name       old speed      new speed       delta
Equal/1     122MB/s ± 0%    121MB/s ± 0%    -0.94%
Equal/2     223MB/s ± 0%    241MB/s ± 0%    +8.29%
Equal/3     289MB/s ± 0%    362MB/s ± 0%   +24.90%
Equal/4     366MB/s ± 0%    483MB/s ± 0%   +31.82%
Equal/5     427MB/s ± 0%    603MB/s ± 0%   +41.28%
Equal/6     462MB/s ± 0%    723MB/s ± 0%   +56.65%
Equal/7     509MB/s ± 0%    843MB/s ± 0%   +65.57%
Equal/8     974MB/s ± 0%   1066MB/s ± 0%    +9.46%
Equal/9    1.00GB/s ± 0%   1.20GB/s ± 0%   +19.53%
Equal/10   1.00GB/s ± 0%   1.33GB/s ± 0%   +32.81%
Equal/11   1.01GB/s ± 0%   1.47GB/s ± 0%   +45.28%
Equal/12   1.04GB/s ± 0%   1.60GB/s ± 0%   +53.46%
Equal/13   1.05GB/s ± 0%   1.73GB/s ± 0%   +64.67%
Equal/14   1.02GB/s ± 0%   1.87GB/s ± 0%   +82.93%
Equal/15   1.04GB/s ± 0%   2.00GB/s ± 0%   +92.07%
Equal/16   1.83GB/s ± 0%   2.13GB/s ± 0%   +16.58%
Equal/17   1.78GB/s ± 0%   2.18GB/s ± 0%   +22.65%
Equal/20   1.72GB/s ± 0%   2.57GB/s ± 0%   +49.24%
Equal/32   3.89GB/s ± 0%   4.10GB/s ± 0%    +5.53%
Equal/63   3.63GB/s ± 0%   7.63GB/s ± 0%  +110.45%
Equal/64   6.69GB/s ± 0%   7.75GB/s ± 0%   +15.84%
Equal/65   6.28GB/s ± 0%   7.07GB/s ± 0%   +12.46%
Equal/127  6.41GB/s ± 0%  13.65GB/s ± 0%  +112.95%
Equal/128  11.1GB/s ± 0%   14.1GB/s ± 0%   +26.56%
Equal/129  10.2GB/s ± 0%   11.2GB/s ± 0%    +9.44%
Equal/191  8.64GB/s ± 0%  16.39GB/s ± 0%   +89.75%
Equal/192  15.3GB/s ± 0%   17.8GB/s ± 0%   +16.31%
Equal/4K   24.6GB/s ± 0%   27.8GB/s ± 0%   +13.12%
Equal/4M   21.1GB/s ± 0%   22.7GB/s ± 0%    +7.66%
Equal/64M  20.8GB/s ± 0%   22.4GB/s ± 0%    +8.06%

Change-Id: Ie3c582133d526cc14e8846ef364c44c93eb7b9a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/399976
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>