[llvm] [VPlan] Remove CanonicalIV when dissolving loop regions (NFC). (PR #142372)

[via llvm-commits]

ayalz wrote:

Post-commit follow-up comment: this raises the thought of coupling `CanonicalIVPHI` with its loop Region - also to be created together, i.e., within a `createFromCFGLoop()` complementing `dissolveToCFGLoop()`. A loop region could hold its CanonicalIVPHI rather than place it inside its header block, abstracting away `CanonicalIVIncrement` along with its cyclic dependence w/o risk of dce, guaranteeing  CanonicalIVPHI's uniqueness.

https://github.com/llvm/llvm-project/pull/142372