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

[Luke Lau via llvm-commits]

lukel97 wrote:

> I've queued up a run for this on the BPI-F3, will have results sometime tomorrow hopefully!

I'm happy to report that there's some fairly confident gains with this PR, 3.27% on on 511.povray_r and 2.76% on 523.xalancbmk_r, both with very little noise: https://lnt.lukelau.me/db_default/v4/nts/328

On povray a good chunk of the speed up comes from the frame setup in `pov::All_Sphere_Intersections` being cheaper now that it no longer uses fs3-fs8. If you view it in the [profile here](https://lnt.lukelau.me/db_default/v4/nts/profile/3/328/329) and select View: Control-Flow Graph (RISC-V), you can see the extra spills inserted instead in the last block, which is very cold.

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