[llvm] [RISCV] Refactor GPRF64 register class to make it usable for Zacas. (PR #77408)

[Shao-Ce SUN via llvm-commits]

sunshaoce wrote:

https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/Sparc/SparcISelDAGToDAG.cpp#L234

I noticed someone used `GPRPair` to describe the `i32 GPR Pair` earlier, does this have any impact?

https://github.com/llvm/llvm-project/pull/77408