Benchmark count for vector<bool>

[AlexGuteniev]

There is count for vector<bool> optimization that uses popcnt on the integer elements of the vector<bool> internal representation, originally added in #1131. There's open PR #5640 to enhance that optimization further.

This PR adds benchmark to measure the results.

I've started off copying vector_bool_copy.cpp to mimic the existing style, then left only the _algined case, since unalignment doesn't make significant impact (unlike copying), still left the same name (as just count matches the STL algorithm name), and added DoNotOptimize where necessary. The value to count is alternating to explore both branches without adding extra benchmarks.

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The results for #5640 are mixed for me.

On P cores of i5-1235U I see no improvement:

Benchmark	Before	After	Speedup
count_aligned/64	17.0 ns	17.0 ns	1.00
count_aligned/4096	61.5 ns	59.6 ns	1.03
count_aligned/65536	718 ns	747 ns	0.96

On E cores I see some improvement, which is not too little for such a small change:

Benchmark	Before	After	Speedup
count_aligned/64	21.3 ns	21.6 ns	0.99
count_aligned/4096	114 ns	90.6 ns	1.26
count_aligned/65536	1505 ns	1092 ns	1.38

[StephanTLavavej]

5950X results:

Benchmark	Before	After	Speedup
count_aligned/64	18.8 ns	18.8 ns	1.00
count_aligned/4096	77.7 ns	62.8 ns	1.24
count_aligned/65536	903 ns	786 ns	1.15

[StephanTLavavej]

Thanks! 😻 I pushed minor stylistic changes.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for measuring this performance! ⏱️ 🚀 😻