Reserve HashSet capacity before inserting cfgs/check-cfgs

[Urgau]

This PR reserve capacity before inserting cfgs/check-cfgs into their different hashsets.

Related to #137005, mostly an experiment, but even if perf is neutral it still a good practice.

[rustbot]

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[Urgau]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 5a01c94 with merge 8c9c6b3...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8c9c6b3 (8c9c6b31e6fac10dd47adacbb9a3398f8f19bd36)

[rust-timer]

Finished benchmarking commit (8c9c6b3): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -2.9%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.9%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -7.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.7%	[1.7%, 1.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.8%	[-9.1%, -8.5%]	6
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 788.977s -> 789.944s (0.12%)
Artifact size: 347.33 MiB -> 347.33 MiB (-0.00%)

[nnethercote]

even if perf is neutral it still a good practice.

I've gotta be honest: I totally disagree with this, and I don't like this approach at all. The comments mention that there are a few dozen cfgs. Resizing those hashmaps is going to be utterly negligible, time-wise. Only the very largest hashmaps, with 10s or 100s of thousands of entries, maybe millions, are going to be worth pre-sizing. It's additional code and comments for zero benefit.

Sorry, I'm going to nip this one in the bud, and comment on #137005.