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

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

OK, I did't mean to be picky here, but please just wait for another run for value 5. We are not in a release merge window so we are not so hurried, right? :-)

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