Use immediate_backend_type when reading from a const alloc

[saethlin]

Fixes #118047

r? @nikic

[nikic]

Do we need a test for this? The errant IR is only found by an assertion, what comes out with --emit=llvm-ir looks like it's been fixed up by something.

"Does not assert" seems like a worthwhile thing to test.

[saethlin]

I've added a ui/codegen test based on my reduced example

[nikic]

@bors r+

[bors]

📌 Commit b0a5801 has been approved by nikic

It is now in the queue for this repository.

[bors]

⌛ Testing commit b0a5801 with merge 7e452c1...

[bors]

☀️ Test successful - checks-actions
Approved by: nikic
Pushing 7e452c1 to master...

[rust-timer]

Finished benchmarking commit (7e452c1): 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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.1%	[-1.2%, -1.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.2%, -1.1%]	2

Binary size

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

Bootstrap: 668.148s -> 668.954s (0.12%)
Artifact size: 314.08 MiB -> 314.14 MiB (0.02%)