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

[Tom Vijlbrief via llvm-commits]

tomtor wrote:

> Huh, weird - from what I've gathered, the issue is that Rust's LLVM fork generates a somewhat longer assembly output, right?

Yes, starting 2025-08-07. But this morning nightly was ok again.

https://github.com/llvm/llvm-project/commit/62735d26b1a1bdb5d03c594c958a0d01a4f8b486 introduced the fix after introducing the bug 2 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..

`mulqi3` was no longer generated for an 8bit modulo operatio, but instead `mulhi3` (which is the longer assembly) because the 8bit value was wrongly extended to 16bit. I have no clue why Rust nightly is wrong again.

This PR just rebased on current llvm and it runs fine.

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