[llvm] [RISCV] Clear vill for whole vector register moves in vsetvli insertion (PR #118283)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Do we need to add `Uses = [VTYPE]` to the whole register move instructions in RISCVInstrInfoV.td?

I think so. I was also thinking that we probably need to somehow add a use of VTYPE to the COPY instructions too. I don't think anything is preventing them from being shuffled about at the moment.

Maybe we need to eagerly lower the copies in RISCVInsertVSETVLI? We could do it in this PR or in a separate one.

https://github.com/llvm/llvm-project/pull/118283