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

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM w/minor comments only.

I am convinced by the performance and insertion count data provided.  

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