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

This transformation might be illegal for `PseduoVIOTA_M`. The value of
`viota.m vd, vs2` is the prefix sum of vd2 and adding mask for it may
cause wrong prefix sum.
Take an example, the result of following expression is `{5, 5, 5, 3}`,
```
; v4 = {1, 1, 1, 1}
viota.m v1, v4
; v0 = {0, 0, 0, 1}, v1 = {0, 1, 2, 3}, v8 = {5, 5, 5, 5}
vmerge.vvm v8, v8, v1, v0.t
; v8 = {5, 5, 5, 3}
```
but if we merge them to `viota.m v8, v4, v0.t`, then the result of is
`{5, 5, 5, 0}`.
Also, we still does `performCombineVMergeAndVOps` for `voita.m` when
mask of `vmerge.vvm` is a true mask.