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

[Matthias Braun via llvm-commits]

MatzeB wrote:

- Not clear to me why RiscV needs another analysis when we already have `LiveIntervals`  (and we also have the old/deprecated `LiveVariables`; this would add a 3rd analysis into the mix...)
- If you want to use this pass anyway: At least consider using the existing `LivePhysRegs` / `LiveRegUnits` helpers to perform liveness analysis within a basic block. They reduce the risk of getting tricky bits like early clobbers or the unintuitive/bad moddeling for return blocks ("pristine registers", ...)

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