math/bits: directly calculate quo/rem when hi is zero in Div64

math/bits: directly calculate quo/rem when hi is zero in Div64

func Div64(hi, lo, y uint64) (quo, rem uint64) {...}

math/bits.Div64 returns the quotient and remainder of (hi, lo) divided
by y. When hi is zero, we can directly return lo/y, lo%y, which can save
a lot of unnecessary calculations.

The performance is measured on arm64 and the changes will only affect
the arch that doesn't have the intrinsic.

name                old time/op    new time/op    delta
DivWVW/1-10         4.62ns ± 1%    2.45ns ± 1%  -46.97%
DivWVW/2-10         12.4ns ± 0%    12.2ns ± 0%   -1.38%
DivWVW/3-10         17.4ns ± 1%    17.2ns ± 0%   -0.88%
DivWVW/4-10         21.4ns ± 1%    21.6ns ± 0%   +0.75%
DivWVW/5-10         22.1ns ± 1%    21.9ns ± 0%   -0.69%
DivWVW/10-10        53.4ns ± 1%    53.0ns ± 1%   -0.69%
DivWVW/100-10        641ns ± 1%     633ns ± 0%   -1.26%
DivWVW/1000-10      5.52µs ± 1%    5.44µs ± 0%   -1.39%
DivWVW/10000-10     54.9µs ± 1%    54.7µs ± 1%   -0.54%
DivWVW/100000-10     646µs ± 1%     643µs ± 1%     ~

name                  old speed      new speed    delta
DivWVW/1-10       13.8GB/s ± 1%  26.1GB/s ± 1%  +88.57%
DivWVW/2-10       10.3GB/s ± 0%  10.5GB/s ± 0%   +1.39%
DivWVW/3-10       11.1GB/s ± 1%  11.2GB/s ± 0%   +0.90%
DivWVW/4-10       12.0GB/s ± 1%  11.9GB/s ± 0%   -0.74%
DivWVW/5-10       14.5GB/s ± 1%  14.6GB/s ± 0%   +0.69%
DivWVW/10-10      12.0GB/s ± 1%  12.1GB/s ± 1%   +0.69%
DivWVW/100-10     10.0GB/s ± 1%  10.1GB/s ± 0%   +1.28%
DivWVW/1000-10    11.6GB/s ± 1%  11.8GB/s ± 0%   +1.41%
DivWVW/10000-10   11.6GB/s ± 1%  11.7GB/s ± 1%   +0.54%
DivWVW/100000-10  9.91GB/s ± 1%  9.95GB/s ± 1%     ~

Change-Id: I12014c2e2cdb2c91608079f7502592307af9e525
Reviewed-on: https://go-review.googlesource.com/c/go/+/449776
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>