Give temporaries in if let guards correct scopes

[matthewjasper]

Temporaries in if-let guards have scopes that escape the match arm, this causes problems because the drops might be for temporaries that are not storage live. This PR changes the scope of temporaries in if-let guards to be limited to the arm:

_ if let Some(s) = std::convert::identity(&Some(String::new())) => {}
//                Temporary for Some(String::new()) is dropped here ^

We also now deduplicate temporaries between copies of the guard created for or-patterns:

// Only create a single Some(String::new()) temporary variable
_ | _ if let Some(s) = std::convert::identity(&Some(String::new())) => {}

This changes MIR building to pass around ExprIds rather than Exprs so that we have a way to index different expressions.

cc #51114
Closes #116079

[rustbot]

r? @TaKO8Ki

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[TaKO8Ki]

@bors rollup=never

[TaKO8Ki]

@bors r+

[bors]

📌 Commit d437a11 has been approved by TaKO8Ki

It is now in the queue for this repository.

[bors]

⌛ Testing commit d437a11 with merge f2348fb...

[bors]

☀️ Test successful - checks-actions
Approved by: TaKO8Ki
Pushing f2348fb to master...

[rust-timer]

Finished benchmarking commit (f2348fb): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

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)	-	-	0
Regressions ❌ (secondary)	2.9%	[2.4%, 3.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.6%, -1.6%]	3
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	3
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	1

Bootstrap: 672.006s -> 669.882s (-0.32%)
Artifact size: 312.63 MiB -> 312.63 MiB (-0.00%)