[llvm] [experimental] Ask copilot to check perf profile degradation (PR #146271)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

Note to reviewers: I didn't want to make the prompt too stuffy, in some local experiments, something this short ended up being reasonably good. I'd rather get going, start experimenting with this for a while by selectively adding it to PRs and seeing what mileage we get, and we can wordsmith it if needed.

https://github.com/llvm/llvm-project/pull/146271