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

[Yeting Kuo via llvm-commits]

yetingk wrote:

> Which presumably means we can't convert from an all ones mask -> unmasked. 

I think we can still does this transformation for `viota.m`, since all ones mask does not influence the prefix sum calculation.

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