[llvm] [RISCV] Refactor subreg indices. (PR #77173)

[Craig Topper via llvm-commits]

topperc wrote:

> I don't think any of these names are an improvement. Why are you of the view Zacas needs this renaming?

Zacas needs a pair of GPRs. sub_32 is clearly not the right name for the lower half a RV64 GPR pair.

Should we make SubReg size and offsets HwMode aware? Or have different regclasses for pairs for  RV32 and RV64

If we make subregs hwmode aware then we definitely need different indices for GPR and FPR.

https://github.com/llvm/llvm-project/pull/77173