[llvm] [VPlan] Replace VPRegionBlock with explicit CFG before execute (NFCI). (PR #117506)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Just pushed a major update of the patch, as most pending improvements needed for this have now landed.

There are ~8 tests still failing/needing updating, but the general changes should be good to review

https://github.com/llvm/llvm-project/pull/117506