Add codegen test to guard against VecDeque optimization regression

[a-lafrance]

Very small PR that adds a codegen test to guard against regression for the VecDeque optimization addressed in #80836. Ensures that Rustc optimizes away the panic when unwrapping the result of .get(0) because of the !is_empty() condition.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit d5ec9af has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit d5ec9af with merge 19c6502...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 19c6502 to master...

[rust-timer]

Finished benchmarking commit (19c6502): comparison URL.

Overall result: ✅ improvements - 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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

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

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: 634.954s -> 634.731s (-0.04%)
Artifact size: 317.84 MiB -> 317.81 MiB (-0.01%)