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

[Luke Lau via llvm-commits]

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

LGTM. For a future patch, could we generalize this with `llvm::isNeutralConstant` so we can handle vwmul too?

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