bitset: Improve performance of count function

[stotko]

The count() function of the bitset container used two distinct reductions for computing the number of set bits. The first one considers all bits in the first n - 1 block whereas the second one works on each of the remaining bits in the n-th block, i.e. the last block. Since the reduction is memory-bound anyways, the second reduction introduces a large overhead while the first one still has room for additional compute operations. Merge both reductions into a single one that computes a bit mask for each block on the fly before counting the bits per block. This leads to a decent performance improvement for count() and all related functions.

Before:

-------------------------------------------------------------------------
Benchmark                               Time             CPU   Iterations
-------------------------------------------------------------------------
stdgpu_bitset_count/1000            0.050 ms        0.051 ms        13822
stdgpu_bitset_count/1000000         0.056 ms        0.056 ms        12476
stdgpu_bitset_count/1000000000      0.733 ms        0.733 ms          956
stdgpu_bitset_all/1000              0.050 ms        0.050 ms        13879
stdgpu_bitset_all/1000000           0.056 ms        0.056 ms        12538
stdgpu_bitset_all/1000000000        0.736 ms        0.734 ms          954
stdgpu_bitset_any/1000              0.050 ms        0.050 ms        13841
stdgpu_bitset_any/1000000           0.056 ms        0.056 ms        12165
stdgpu_bitset_any/1000000000        0.734 ms        0.734 ms          954
stdgpu_bitset_none/1000             0.050 ms        0.051 ms        13811
stdgpu_bitset_none/1000000          0.056 ms        0.056 ms        12428
stdgpu_bitset_none/1000000000       0.735 ms        0.734 ms          954

After:

-------------------------------------------------------------------------
Benchmark                               Time             CPU   Iterations
-------------------------------------------------------------------------
stdgpu_bitset_count/1000            0.026 ms        0.026 ms        26891
stdgpu_bitset_count/1000000         0.031 ms        0.031 ms        22745
stdgpu_bitset_count/1000000000      0.580 ms        0.580 ms         1208
stdgpu_bitset_all/1000              0.026 ms        0.026 ms        26856
stdgpu_bitset_all/1000000           0.031 ms        0.031 ms        22795
stdgpu_bitset_all/1000000000        0.580 ms        0.580 ms         1207
stdgpu_bitset_any/1000              0.026 ms        0.026 ms        26940
stdgpu_bitset_any/1000000           0.031 ms        0.031 ms        22814
stdgpu_bitset_any/1000000000        0.580 ms        0.580 ms         1207
stdgpu_bitset_none/1000             0.026 ms        0.026 ms        26945
stdgpu_bitset_none/1000000          0.032 ms        0.032 ms        22725
stdgpu_bitset_none/1000000000       0.581 ms        0.580 ms         1185

[codecov]

Codecov Report

Merging #309 (b2f655f) into master (2e13b2e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   97.47%   97.47%   -0.01%     
==========================================
  Files          32       32              
  Lines        2300     2294       -6     
==========================================
- Hits         2242     2236       -6     
  Misses         58       58              

Impacted Files	Coverage Δ
src/stdgpu/impl/bitset_detail.cuh	100.00% <100.00%> (ø)

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