[llvm] [RISCV][RegAlloc] Add getCSRFirstUseCost for RISC-V (PR #131349)

[Philip Reames via llvm-commits]

preames wrote:

One other question.
3) What are the noise bands on your runtime measurements?  Glancing at e.g. x264 which has basically no dynamic IC change, the runtime measurements look a lot like random noise.  That's somewhat unavoidable when the impact is this small, but I want to have some clue which numbers I should be treating as having likely statistical significance.  

@lukel97 Might be good to queue this one on the BP3.  Your setup does a decent job of pruning statistical false positives, and as above, it looks like that might be relevant here.

https://github.com/llvm/llvm-project/pull/131349