math/bits: faster OnesCount, added respective benchmarks

math/bits: faster OnesCount, added respective benchmarks

Also: Changed Reverse/ReverseBytes implementations to use
the same (smaller) masks as OnesCount.

BenchmarkOnesCount-8          37.0          6.26          -83.08%
BenchmarkOnesCount8-8         7.24          1.99          -72.51%
BenchmarkOnesCount16-8        11.3          2.47          -78.14%
BenchmarkOnesCount32-8        18.4          3.02          -83.59%
BenchmarkOnesCount64-8        40.0          3.78          -90.55%
BenchmarkReverse-8            6.69          6.22          -7.03%
BenchmarkReverse8-8           1.64          1.64          +0.00%
BenchmarkReverse16-8          2.26          2.18          -3.54%
BenchmarkReverse32-8          2.88          2.87          -0.35%
BenchmarkReverse64-8          5.64          4.34          -23.05%
BenchmarkReverseBytes-8       2.48          2.17          -12.50%
BenchmarkReverseBytes16-8     0.63          0.95          +50.79%
BenchmarkReverseBytes32-8     1.13          1.24          +9.73%
BenchmarkReverseBytes64-8     2.50          2.16          -13.60%

OnesCount-8       37.0ns ± 0%   6.3ns ± 0%   ~             (p=1.000 n=1+1)
OnesCount8-8      7.24ns ± 0%  1.99ns ± 0%   ~             (p=1.000 n=1+1)
OnesCount16-8     11.3ns ± 0%   2.5ns ± 0%   ~             (p=1.000 n=1+1)
OnesCount32-8     18.4ns ± 0%   3.0ns ± 0%   ~             (p=1.000 n=1+1)
OnesCount64-8     40.0ns ± 0%   3.8ns ± 0%   ~             (p=1.000 n=1+1)
Reverse-8         6.69ns ± 0%  6.22ns ± 0%   ~             (p=1.000 n=1+1)
Reverse8-8        1.64ns ± 0%  1.64ns ± 0%   ~     (all samples are equal)
Reverse16-8       2.26ns ± 0%  2.18ns ± 0%   ~             (p=1.000 n=1+1)
Reverse32-8       2.88ns ± 0%  2.87ns ± 0%   ~             (p=1.000 n=1+1)
Reverse64-8       5.64ns ± 0%  4.34ns ± 0%   ~             (p=1.000 n=1+1)
ReverseBytes-8    2.48ns ± 0%  2.17ns ± 0%   ~             (p=1.000 n=1+1)
ReverseBytes16-8  0.63ns ± 0%  0.95ns ± 0%   ~             (p=1.000 n=1+1)
ReverseBytes32-8  1.13ns ± 0%  1.24ns ± 0%   ~             (p=1.000 n=1+1)
ReverseBytes64-8  2.50ns ± 0%  2.16ns ± 0%   ~             (p=1.000 n=1+1)

Change-Id: I591b0ffc83fc3a42828256b6e5030f32c64f9497
Reviewed-on: https://go-review.googlesource.com/37218
Reviewed-by: Matthew Dempsky <mdempsky@google.com>