[llvm] [RISCV] Fold PseudoVMV_V_V with undef passthru, handling policy (PR #106943)

[Luke Lau via llvm-commits]

lukel97 wrote:

I've reworked this PR and updated the description to do this as a separate peephole, as I think it's easier to understand without having to think about foldVMV_V_V.

https://github.com/llvm/llvm-project/pull/106943