[llvm] [AVR] Add extra codegen test (PR #152902)

[Trevor Gross via llvm-commits]

tgross35 wrote:

> [62735d2](https://github.com/llvm/llvm-project/commit/62735d26b1a1bdb5d03c594c958a0d01a4f8b486) introduced the fix after introducing the bug 3 days earlier (see commit messsge), so it is not a real Rust issue, it is in the llvm used by Rust nightly, which apparently is very up-to-date..

Rustc is pretty up to date - usually the major version bumps get merged as soon as we know LLVM's release date will be before Rust's stable release date for that cycle (after a few months of testing), and minor bumps happen immediately. But that commit and the one that fixed it are still too new to have made it to our fork https://github.com/rust-lang/llvm-project/commits/rustc/21.1-2025-08-01/.

Maybe it regressed with the llvm bump but got fixed with a more codegen change on Rust's side?

https://github.com/llvm/llvm-project/pull/152902