coverage: Branch coverage support for let-else and if-let

[Zalathar]

This PR adds branch coverage instrumentation for let-else and if-let, including let-chains.

This lifts two of the limitations listed at #124118.

[Zalathar]

Record if-let branches a bit later, to avoid unwrapping anything (diff).

[Zalathar]

Originally I was holding this back until after #124154, both to make branch management easier, and to see whether further work on that PR would prompt changes to this one.

But while the future of #124154 remains a bit murky, I think it now makes sense to merge this PR as an independent improvement in its own right.

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[Zalathar]

r? compiler

[Zalathar]

After a rebase, I reintroduced an intermediate method instead of calling add_two_way_branch directly (diff).

This is indeed a nice improvement. In hindsight, a lot of my reasons for having done things the other way have now evaporated.

[bors]

☔ The latest upstream changes (presumably #124399) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r+

[bors]

📌 Commit 7c87ad0 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7c87ad0 with merge 5486f0c...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 5486f0c to master...

[rust-timer]

Finished benchmarking commit (5486f0c): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Max RSS (memory usage)

Results

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)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.0%	[-4.0%, -4.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.4%	[-4.0%, 1.1%]	2

Cycles

Results

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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	4.1%	[4.1%, 4.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Binary size

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

Bootstrap: 677.065s -> 676.569s (-0.07%)
Artifact size: 315.85 MiB -> 315.94 MiB (0.03%)