[llvm] [VPlan] Delay adding canonical IV increment. (PR #82270)

[via llvm-commits]

https://github.com/ayalz commented:

This raises several thoughts, including the one commented at the outset - of representing the canonical IV and active lane mask as VPValues of the VPRegionBlock rather than ordinary header phi recipes of the header VPBasicBlock.

https://github.com/llvm/llvm-project/pull/82270