[RISC-V] Zilsd Improvements

I thought it might be good to centralise the TODO list for Zilsd support, so we can collaborate across orgs a little better.

Here are some possible improvements relating to Zilsd:
- [ ] Spills/Reloads (https://github.com/llvm/llvm-project/pull/153595)
- [ ] Callee-Saves?
- [ ] Allocation Priority of GPRPairs?

Craig remarked there's an interaction of Zilsd with Zfinx/Zdinx which we may need to resolve/improve. I don't have the details fully in my head.

NXP's tree with Zilsd support is here: https://github.com/nxp-auto-tools/llvm-project/tree/Zilsd/release/18.1.6 - but much of these changes have been reimplemented from scratch upstream already.

Start by reviewing the listed Zilsd work, especially PR 153595, and compare it with NXP's Zilsd branch. The issue is complete only when the proposed spill/reload, callee-save, register-pair allocation, and Zfinx/Zdinx interaction improvements have defined resolutions, so further design discussion is needed first.

Written by the indexing model from the issue text.