[llvm] [RISCV][ISel] Remove redundant vmerge for vwsub(u).wv. (PR #80523)

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sun-jacobi wrote:

> > LGTM. For a future patch, could we generalize this with `llvm::isNeutralConstant` so we can handle vwmul too?
> 
> Isn't this patch for .wv instructions? There is no vwmul.wv.

oops, you are right. Thanks

https://github.com/llvm/llvm-project/pull/80523