perf: don't needlessly trigger bitcount

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 :tada:

Comparison is base (e923e03) 83.46% compared to head (e71d666) 83.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1513 +/- ## ========================================== + Coverage 83.46% 83.50% +0.04% ========================================== Files 388 388 Lines 42012 42024 +12 ========================================== + Hits 35067 35094 +27 + Misses 6945 6930 -15 ``` | [Impacted Files](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao) | Coverage Δ | | |---|---|---| | [src/bitmap/immutable.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2JpdG1hcC9pbW11dGFibGUucnM=) | `85.84% <100.00%> (+6.21%)` | :arrow_up: | | [src/bitmap/mutable.rs](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1513?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao#diff-c3JjL2JpdG1hcC9tdXRhYmxlLnJz) | `96.99% <100.00%> (+0.05%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/jorgecarleitao/arrow2/pull/1513/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jorge+Leitao)

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