[llvm] [RISCV] Merge GPRPair and GPRF64Pair (PR #116094)

[Sam Elliott via llvm-commits]

lenary wrote:

> > @topperc what do you think about this approach? Do you think I should be working harder to merge `BuildGPRPair` and `BuildPairF64` (and similarly for Split)
> 
> I'm ok with them being separate. It gives some degree of type safety.

Cool.

I'm not sure there's much more to do, but maybe I missed something?

https://github.com/llvm/llvm-project/pull/116094