[llvm] [RISCV] Select mask operands as virtual registers and eliminate vmv0 (PR #125026)

[Philip Reames via llvm-commits]

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

LGTM w/comments addressed.

Though, I'd encourage you to wait on review from @topperc .  He's better at spotting subtle problems in this type of thing than I am.

https://github.com/llvm/llvm-project/pull/125026