[llvm] [MCA] use std::function instead of function_ref when storing (PR #91039)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Doing some quick microbenchmarks, I'm only seeing a 2-3% performance regression for computing the cost of 10000 instructions, so I think fixing the correctness/API design issue is probably worth the performance cost. I'm going to land this and we can revisit this if the performance regression causes issues for people/or someone is willing to implement a lightweight function reference for this situation.

https://github.com/llvm/llvm-project/pull/91039