Fix MaybeUninit codegen using GVN

[saethlin]

This is an alternative to #142837, based on #146355 (comment).

[saethlin]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e6fd12c (e6fd12c9ca883127b096afd871b40a5714f3d107, parent: f46475914de626785090a05ae037578aaa119fc8)

[rust-timer]

Finished benchmarking commit (e6fd12c): 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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	3
Improvements ✅ (primary)	-7.4%	[-7.4%, -7.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-7.4%, 0.7%]	2

Max RSS (memory usage)

Results (primary 0.9%, secondary 0.8%)

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

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	3.0%	[1.8%, 4.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	0.9%	[0.9%, 0.9%]	1

Cycles

Results (primary -3.6%)

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)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-3.6%, -3.6%]	1

Binary size

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

Bootstrap: 475.372s -> 474.527s (-0.18%)
Artifact size: 390.39 MiB -> 390.41 MiB (0.00%)