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

[Pengcheng Wang via llvm-commits]

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

The result is promising! One concern is that I don't know if we have set the cost too high. What is the logic chain after setting the value of this cost? Definitely it is not higher is better, but what is the rule to balance it?
Also I don't see the big improvement on 523.xalancbmk_r on SiFive core, which makes me hesitate about the value 64.

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