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

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Minor comments.  This approach looks entirely reasonable, will be happy to LGTM if the perf results look good.  

Note that I remain concerned that something after VSETVLI insertion will move/insert a copy.  This probably covers most of the cases, but I'm not convinced this completely closes the correctness hole.  

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