[llvm] [llvm-exegesis] Add support for warmup iterations (PR #76895)

[Clement Courbet via llvm-commits]

legrosbuffle wrote:

> This patch additionally contains two refactorings that could be moved into separate patches 

Yes, if you could do that that would make the review easier. Thanks.

https://github.com/llvm/llvm-project/pull/76895