[llvm] [RISCV] Optimise spills/fills of FPR<->GPR moves (PR #78408)

[Alex Bradbury via llvm-commits]

asb wrote:

Thank you both for catching those issues - I made the mistake of thinking "oh I should just add the X<->W case too" and clearly didn't engage my brain enough. I'll rework.

https://github.com/llvm/llvm-project/pull/78408