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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

It all makes sense to me, I just wonder if the value 64 is too high since the value on AArch64 is 5. I don' see any big arch difference between RISC-V and AArch64 since they are both arch for RISC CPUs.

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