[llvm] [RISCV] Fix vmerge.vvm/vmv.v.v getting folded into ops with mismatching EEW (PR #101152)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: d01c0514ab272bc06aa5845acb810d0eec587436

https://github.com/llvm/llvm-project/actions/runs/10166340246

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/101152