[clang-tools-extra] [clang] [llvm] [RISCV] Eliminate dead li after emitting VSETVLIs (PR #65934)

[Luke Lau via cfe-commits]

lukel97 wrote:

> > Oh nice, I'm running into something similar in #71657. Are all these dead ADDIs coming from the backwards local postpass?
> 
> Yes. I believe this PR can address the issue.

My suspicion is that these LIs only become dead because the backwards local postpass deletes a vsetvli that had an ADDI AVL. Would it be easier then to just check if they are dead in `doLocalPostpass` rather than keeping around a vector of instructions? I.e. something similar to https://github.com/llvm/llvm-project/pull/71657/commits/8ae868b3825ab5c2d9506c2ca515687d67419b9b

https://github.com/llvm/llvm-project/pull/65934