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

[via llvm-commits]

hiraditya wrote:

> @hiraditya I took a look to try to benchmark this - I'm not able to build SPEC, multiple failures of:
> ```
> Computed live-in set is inconsistent with MBB.
> UNREACHABLE executed at ../../llvm/lib/Target/RISCV/RISCVLiveVariables.cpp:443!
> ```
> 
> I'll attempt to get a minimised test case for you.

Oh this one is because the current license information is stale. I wrote the verifier to prove that. We can either:

 1. disable verifier (-mllvm -riscv-liveness-verify), more conservative approach as it keeps the existing basic block live in/outs
2. Enable (-mllvm -riscv-liveness-update-mbb-liveness) to update the basic block live in/out sets.

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