[llvm] [VPlan] Fix header phi VPInstruction verification. NFC (PR #151472)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Can you add a test with a non-header-phi in header block to `llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp`?

I guess at the moment this would just be `VPPredInstPHI`?

https://github.com/llvm/llvm-project/pull/151472