mir-opt: Remove the workaround in UnreachableEnumBranching

[dianqk]

#120268 added a workaround due to the compile time of TailDuplicator.

LLVM 20 has resolved this in llvm/llvm-project#114990 and llvm/llvm-project#132536.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @petrochenkov

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[dianqk]

cc @nikic because of #120268 (comment)

[workingjubilee]

I believe you meant to pingback LLVM issues, not Rust issues?

[nikic]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c2e723d (c2e723d18a7db9f726e1b5b93246eb112aadd922, parent: 52618eb338609df44978b0ca4451ab7941fd1c7a)

[rust-timer]

Finished benchmarking commit (c2e723d): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (secondary 3.7%)

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)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	1

Bootstrap: 474.182s -> 473.61s (-0.12%)
Artifact size: 390.15 MiB -> 390.16 MiB (0.00%)

[dianqk]

I believe you meant to pingback LLVM issues, not Rust issues?

I linked back to the PR that introduced the workaround.

[petrochenkov]

r? @nikic

[wesleywiser]

r? wesleywiser

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.