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

[Philip Reames via llvm-commits]

https://github.com/preames commented:

As we talked about last night, very supportive of this direction.  I think this approach works out.

One suggestion: can we stage this into two commits to make the diffs easier to understand?  If you do the new pass immediately after ISEL, I think you get something which is much closer to NFC.  This lets us land the majority of the cleanup  Then you can do a separate change which moves it before regalloc, and we can focus on the interaction with all the passes being moved over.  

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