math/bits: folded reverse tables by using const string

math/bits: folded reverse tables by using const string

linux/mips64le

name            old time/op  new time/op  delta
Reverse         2.31ns ± 1%  2.27ns ± 1%  -1.53%  (p=0.001 n=10+10)
Reverse8        0.65ns ± 1%  0.65ns ± 1%  -1.19%  (p=0.000 n=9+10)
Reverse16       1.15ns ± 2%  1.14ns ± 2%    ~     (p=0.062 n=9+10)
Reverse32       1.96ns ± 1%  1.94ns ± 1%  -1.16%  (p=0.000 n=10+9)
Reverse64       2.29ns ± 1%  2.26ns ± 0%  -0.94%  (p=0.000 n=9+9)
ReverseBytes    0.66ns ± 3%  0.65ns ± 1%  -1.58%  (p=0.006 n=9+10)
ReverseBytes16  0.66ns ± 2%  0.65ns ± 1%  -2.05%  (p=0.000 n=10+9)
ReverseBytes32  0.41ns ± 1%  0.40ns ± 0%  -1.68%  (p=0.000 n=10+10)
ReverseBytes64  0.66ns ± 1%  0.65ns ± 1%  -1.50%  (p=0.000 n=10+9)

cpu=1 benchtime=100ms count=100

name            old time/op  new time/op  delta
Reverse         28.0ns ± 3%  27.7ns ± 3%   -0.80%  (p=0.000 n=100+98)
Reverse8        2.24ns ± 1%  2.24ns ± 1%     ~     (p=0.142 n=98+100)
Reverse16       4.07ns ± 3%  4.05ns ± 3%   -0.66%  (p=0.000 n=99+99)
Reverse32       11.3ns ± 0%  11.3ns ± 0%     ~     (p=0.283 n=94+97)
Reverse64       12.6ns ± 0%  12.6ns ± 0%   +0.60%  (p=0.000 n=100+98)
ReverseBytes    5.25ns ± 1%  5.24ns ± 1%   -0.18%  (p=0.000 n=100+100)
ReverseBytes16  2.00ns ± 0%  2.21ns ± 3%  +10.07%  (p=0.000 n=88+100)
ReverseBytes32  4.08ns ± 2%  4.13ns ± 2%   +1.39%  (p=0.000 n=99+99)
ReverseBytes64  5.48ns ± 1%  5.45ns ± 1%   -0.50%  (p=0.000 n=98+99)

Update #43403

Change-Id: I7e7e00bb17608739d9f6b927c6dfef2580493a0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/280645
Trust: Meng Zhuo <mzh@golangcn.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>