[llvm] [VPlan] Tighten m_CanonicalIV() (PR #194312)

[Florian Hahn via llvm-commits]

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fhahn wrote:

To test this, you should be able to just allocate a new VPRegionValue in the test and match it. With that, it should be possible to test and address the TODO

https://github.com/llvm/llvm-project/pull/194312