[llvm] [VPlan] Use vputils::findCanonicalIVIncrement in more places. NFC (PR #226516)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Do you already have concrete pointers as to in which cases this would be needed in the future?

In general, I think it would be preferable to rely on the structure instead of this lookup unless absolutely not possible

https://github.com/llvm/llvm-project/pull/226516