[llvm] [VPlan] Make CanIV part of region. (PR #144803)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

This should be in pretty good shape, still most printing tests needs to be updated.

This now depends on https://github.com/llvm/llvm-project/pull/155301, to be able to track the live-ness of the canonical IV accurartely when estimating register pressure

https://github.com/llvm/llvm-project/pull/144803