[llvm] [RISCV] Handle undef passthrus in foldVMV_V_V (PR #106943)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Looks functionally correct, but would be much more cleanly expressed as an early return.  This is really just a different transform entirely.  

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