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

[Florian Hahn via llvm-commits]

fhahn wrote:

Made a minor update after  #112138: `getMiddleBlock` now uses `getScalarPreheader()->getPredecessors().front()` instead of `getScalarPreheader()->getSinglePredecessor()`, while before it was getting the middle block from the successor of the vector loop region

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