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

[Yeting Kuo via llvm-commits]

yetingk wrote:

Sorry, I was too busy. I misunderstood it and your comment was actually very clear. 
>  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 your idea is great.

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