[llvm] [RISCV] Implement getIPRACSRegs hook (PR #125586)

[Alex Bradbury via llvm-commits]

asb wrote:

> LGTM. Should we backport to LLVM 20?

Especially this early in the cycle, no opposition to it. I will note that due to #119556 there's at least one other issue with using IPRA in practice - you can get crashes unless you use `-enable-machine-outliner=never`.

https://github.com/llvm/llvm-project/pull/125586