[llvm] [RISCV64] liveness analysis (PR #167454)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Maintaining yet another liveness system is a very substantial maintenance burden.

- This should not be RISCV specific, from the beginning
- There should be a concerted effort to delete the existing LiveVariables

https://github.com/llvm/llvm-project/pull/167454