Avoid the would-change check for bitset chunks that are unique

[Zalathar]

• Successor to Avoid the would-change check for bitset chunks that are unique #153680

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ChunkedBitSet goes out of its way to check in advance whether a union/subtract/intersect operation would actually modify any bits, to avoid calling Rc::make_mut if possible.

But in the case where the Rc is already unique (i.e. Rc::get_mut succeeds), it's cheaper to skip the would-change check entirely, and just modify the words eagerly.

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This PR also adds some more thorough tests for ChunkedBitSet's union/subtract/intersect methods, which would have pinpointed a serious bug in the previous PR.

[Zalathar]

Let's try benchmarking a version of this change that doesn't accidentally skip all the important work:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 047dc4e (047dc4e3e9b4059fc6e425c531cad1c42ca17e54, parent: d1ee5e59a964a419b84b760812a35075034f4861)

[rust-timer]

Finished benchmarking commit (047dc4e): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	2.1%	[1.1%, 3.3%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[-0.2%, 3.3%]	6

Max RSS (memory usage)

Results (secondary 1.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 480.488s -> 479.262s (-0.26%)
Artifact size: 394.87 MiB -> 394.89 MiB (0.00%)

[Zalathar]

Oh, that's disappointing. Perhaps the vectorized equality test is actually faster than an unconditional update, on average.