[llvm] [RISCV] Disable performCombineVMergeAndVOps for PseduoVIOTA_M. (PR #71483)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I wonder if we should add a bit to `RISCVMaskedPseudo` so that we can mark pseudos where changing the enabled elements affects semantics? I think the reduction instructions are in the same boat, except we don't mark it with RISCVMaskedPseudo at all. Which presumably means we can't convert from an all ones mask -> unmasked

https://github.com/llvm/llvm-project/pull/71483