[llvm] [RISCV] Fold PseudoVMV_V_V with undef passthru (PR #106840)

[Luke Lau via llvm-commits]

lukel97 wrote:

After thinking about this a bit, the main difference with this and foldVMV_V_V is that this handles the case where a vmv.v.v has an input that's not another vector pseudo. I'm not sure how often this happens in practice, so I'm closing for now.

https://github.com/llvm/llvm-project/pull/106840