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

[Luke Lau via llvm-commits]

lukel97 wrote:

[The run on SPEC finished on the spacemit-x60, there were no significant changes](https://lnt.lukelau.me/db_default/v4/nts/48). This was with -mcpu=spacemit-x60 -O3 -flto on the train dataset. 523.xalancbmk_r is a bit noisy and I didn't see any significant vector related codegen changes in the diff of the binary

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