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

[Luke Lau via llvm-commits]

lukel97 wrote:

> 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`?

I added a test case in c170996dca9e193166cba01c539c426a30d6c10f before I saw your comment, I think VPIRPhis are considered a non-header-phi too.

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