[llvm] [NFC][AMDGPU][VOPD] Add lit test demonstrating current VOPD pairing behaviour (PR #201943)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Just a nit: does gfx9 behave differently? I am asking because there's a PR that depends on this, and I don't know if that PR's behavior is consistent across gfx12 and gfx9.

GFX9 does not have VOPD so the problem as described would not affect it.

https://github.com/llvm/llvm-project/pull/201943