runtime: simplify divmagic for span calculations

It's both simpler and faster to just unconditionally do two 32-bit
multiplies rather than a bunch of branching to try to avoid them.
This is safe thanks to the tight bounds derived in [1] and verified
during mksizeclasses.go.

Benchstat results below for compilebench benchmarks on my P920. See
also [2] for micro benchmarks comparing the new functions against the
originals (as well as several basic attempts at optimizing them).

name                      old time/op       new time/op       delta
Template                        295ms ± 3%        290ms ± 1%  -1.95%  (p=0.000 n=20+20)
Unicode                         113ms ± 3%        110ms ± 2%  -2.32%  (p=0.000 n=21+17)
GoTypes                         1.78s ± 1%        1.76s ± 1%  -1.23%  (p=0.000 n=21+20)
Compiler                        119ms ± 2%        117ms ± 4%  -1.53%  (p=0.007 n=20+20)
SSA                             14.3s ± 1%        13.8s ± 1%  -3.12%  (p=0.000 n=17+20)
Flate                           173ms ± 2%        170ms ± 1%  -1.64%  (p=0.000 n=20+19)
GoParser                        278ms ± 2%        273ms ± 2%  -1.92%  (p=0.000 n=20+19)
Reflect                         686ms ± 3%        671ms ± 3%  -2.18%  (p=0.000 n=19+20)
Tar                             255ms ± 2%        248ms ± 2%  -2.90%  (p=0.000 n=20+20)
XML                             335ms ± 3%        327ms ± 2%  -2.34%  (p=0.000 n=20+20)
LinkCompiler                    799ms ± 1%        799ms ± 1%    ~     (p=0.925 n=20+20)
ExternalLinkCompiler            1.90s ± 1%        1.90s ± 0%    ~     (p=0.327 n=20+20)
LinkWithoutDebugCompiler        385ms ± 1%        386ms ± 1%    ~     (p=0.251 n=18+20)
[Geo mean]                      512ms             504ms       -1.61%

name                      old user-time/op  new user-time/op  delta
Template                        286ms ± 4%        282ms ± 4%  -1.42%  (p=0.025 n=21+20)
Unicode                         104ms ± 9%        102ms ±14%    ~     (p=0.294 n=21+20)
GoTypes                         1.75s ± 3%        1.72s ± 2%  -1.36%  (p=0.000 n=21+20)
Compiler                        109ms ±11%        108ms ± 8%    ~     (p=0.187 n=21+19)
SSA                             14.0s ± 1%        13.5s ± 2%  -3.25%  (p=0.000 n=16+20)
Flate                           166ms ± 4%        164ms ± 4%  -1.34%  (p=0.032 n=19+19)
GoParser                        268ms ± 4%        263ms ± 4%  -1.71%  (p=0.011 n=18+20)
Reflect                         666ms ± 3%        654ms ± 4%  -1.77%  (p=0.002 n=18+20)
Tar                             245ms ± 5%        236ms ± 6%  -3.34%  (p=0.000 n=20+20)
XML                             320ms ± 4%        314ms ± 3%  -2.01%  (p=0.001 n=19+18)
LinkCompiler                    744ms ± 4%        747ms ± 3%    ~     (p=0.627 n=20+19)
ExternalLinkCompiler            1.71s ± 3%        1.72s ± 2%    ~     (p=0.149 n=20+20)
LinkWithoutDebugCompiler        345ms ± 6%        342ms ± 8%    ~     (p=0.355 n=20+20)
[Geo mean]                      484ms             477ms       -1.50%

[1] Daniel Lemire, Owen Kaser, Nathan Kurz. 2019. "Faster Remainder by
Direct Computation: Applications to Compilers and Software Libraries."
https://arxiv.org/abs/1902.01961

[2] https://github.com/mdempsky/benchdivmagic

Change-Id: Ie4d214e7a908b0d979c878f2d404bd56bdf374f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/300994


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>