[llvm] CodeGen/X86: add missing mcpu to tests (PR #111865)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> The default x86 cpu / tuning attributes are rather messy - I'm not sure hacking the tests is the best way to solve this

Yes, I realized that when updating the tests. However, would we rather have a patch that's improving the schedule in the fallback case generate a ton of test changes?

https://github.com/llvm/llvm-project/pull/111865