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

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

This LGTM. It's a good refactoring, and I think the in-tree testing for Dinx is better than it was.

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