GH-48897: [C++] Add benchmark for CountSetBits

[pitrou]

Rationale for this change

Counting the set bits in a null bitmap is an operation that comes often, it can be useful to get a more precise idea of its performance.

What changes are included in this PR?

Add a benchmark for CountSetBits.

Local results (AMD Zen 2):

------------------------------------------------------------------------------
Benchmark                    Time             CPU   Iterations UserCounters...
------------------------------------------------------------------------------
CountSetBits/16           5.42 ns         5.42 ns    128549068 bytes_per_second=2.7515Gi/s
CountSetBits/1024         80.6 ns         80.6 ns      8649312 bytes_per_second=11.8351Gi/s
CountSetBits/131072       9092 ns         9091 ns        77115 bytes_per_second=13.4271Gi/s

Local results (Intel(R) Core(TM) Ultra 7 255H):

------------------------------------------------------------------------------
Benchmark                    Time             CPU   Iterations UserCounters...
------------------------------------------------------------------------------
CountSetBits/16           2.45 ns         2.45 ns    285392946 bytes_per_second=6.08012Gi/s
CountSetBits/1024         28.9 ns         28.9 ns     23618777 bytes_per_second=33.0086Gi/s
CountSetBits/131072       3490 ns         3489 ns       198472 bytes_per_second=34.9862Gi/s

Are these changes tested?

By running said benchmark manually (and by Continuous Benchmarking).

Are there any user-facing changes?

No.

• GitHub Issue: [C++] Add benchmark for CountSetBits #48897

[pitrou]

@wgtmac @zanmato1984

[pitrou]

Also @AntoinePrv FYI