[llvm] [RISCV] Add test for copy propagation issue with VMV0. NFC (PR #75347)

[Luke Lau via llvm-commits]

lukel97 wrote:

After some discussion with others, it looks using singleton register classes (i.e. VMV0) opens up a whole can of worms, and RISC-V would likely be the only target to extensively do so. It's probably not worth tackling these issues, so closing for now

https://github.com/llvm/llvm-project/pull/75347