[llvm] [VPlan] Use opaque pointers in VPlan unit test IR (PR #69947)

[via llvm-commits]

calebwat wrote:

@fhahn When you have a moment, could you review this please? It's a test-only, nonfunctional change to switch uses of non-opaque pointers to opaque pointers in a couple files.

https://github.com/llvm/llvm-project/pull/69947