[llvm] [VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (PR #114292)

[via llvm-commits]

https://github.com/ayalz approved this pull request.

This LGTM, thanks!

Added few last minor comments; tests seems to also reflect the patch converting canonical and EVL header phi's to scalar header phi's(?).

https://github.com/llvm/llvm-project/pull/114292