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

[Philip Reames via llvm-commits]

preames wrote:

> > Maybe @lukel97 you can issue another several runs with different costs?
> 
> Oh I see what you mean, I'll kick off a run with 5 and see how that compares

I want to be careful that we don't make perfection the enemy of the good here.  We have measured results - both icount and runtime - that show the current number produces reasonable results.  I encourage a data driven sweep here, but I propose we should not spend too much time on this before picking something reasonable (64 seems reasonable), and landing it.  We can continue to sweep the space for a better overall result, but let's take the win we have rather than trying to find the global maxima.  We can always revise the number in the future.

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