math/big: rewrite addVW to use fast path on s390x

math/big: rewrite addVW to use fast path on s390x

Rewrite addVW to use a fast path and remove the original
vector and non vector implementation of addVW in assembly. This CL uses
a similar idea as CL 164968, where we copy the rest of words when we
know carry bit is zero.

In addition, since we are copying vector of words, a faster
implementation of copy is written in this CL to copy a word or multiple
words at a time.

Benchmarks:
name             old time/op    new time/op     delta
AddVW/1-18         4.56ns ± 0%     4.01ns ± 6%   -12.14%  (p=0.000 n=18+20)
AddVW/2-18         5.54ns ± 0%     4.42ns ± 5%   -20.20%  (p=0.000 n=18+20)
AddVW/3-18         6.55ns ± 0%     4.61ns ± 0%   -29.62%  (p=0.000 n=16+18)
AddVW/4-18         6.11ns ± 2%     5.12ns ± 6%   -16.19%  (p=0.000 n=20+20)
AddVW/5-18         7.32ns ± 4%     5.14ns ± 0%   -29.77%  (p=0.000 n=20+19)
AddVW/10-18        10.6ns ± 2%      7.2ns ± 1%   -31.47%  (p=0.000 n=20+20)
AddVW/100-18       49.6ns ± 2%     18.0ns ± 0%   -63.63%  (p=0.000 n=20+20)
AddVW/1000-18       465ns ± 3%      244ns ± 0%   -47.54%  (p=0.000 n=20+20)
AddVW/10000-18     4.99µs ± 4%     2.97µs ± 0%   -40.54%  (p=0.000 n=20+20)
AddVW/100000-18    48.3µs ± 3%     30.8µs ± 1%   -36.29%  (p=0.000 n=20+20)
[Geo mean]         58.1ns          38.0ns        -34.57%

name             old speed      new speed       delta
AddVW/1-18       1.76GB/s ± 0%   2.00GB/s ± 6%   +14.04%  (p=0.000 n=20+20)
AddVW/2-18       2.89GB/s ± 0%   3.63GB/s ± 5%   +25.55%  (p=0.000 n=18+20)
AddVW/3-18       3.66GB/s ± 0%   5.21GB/s ± 0%   +42.25%  (p=0.000 n=18+19)
AddVW/4-18       5.24GB/s ± 2%   6.27GB/s ± 6%   +19.61%  (p=0.000 n=20+20)
AddVW/5-18       5.47GB/s ± 4%   7.78GB/s ± 0%   +42.28%  (p=0.000 n=20+18)
AddVW/10-18      7.55GB/s ± 2%  11.04GB/s ± 1%   +46.09%  (p=0.000 n=20+20)
AddVW/100-18     16.1GB/s ± 2%   44.3GB/s ± 0%  +174.77%  (p=0.000 n=20+20)
AddVW/1000-18    17.2GB/s ± 3%   32.8GB/s ± 1%   +90.58%  (p=0.000 n=20+20)
AddVW/10000-18   16.0GB/s ± 4%   26.9GB/s ± 0%   +68.11%  (p=0.000 n=20+20)
AddVW/100000-18  16.6GB/s ± 3%   26.0GB/s ± 1%   +56.94%  (p=0.000 n=20+20)
[Geo mean]       7.03GB/s       10.75GB/s        +52.93%

Change-Id: Idbb73f3178311bd2b18a93bdc1e48f26869d2f6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/209679
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>