[llvm] [VPlan] Retain exit conditions and edges in initial VPlan (NFC). (PR #137709)

[via llvm-commits]

https://github.com/ayalz commented:

Commit message needs updating (and completion..)

TODO: Suspect `mergeBlocksIntoPredecessors()` might lose predecessor positions as it doesn't check if the empty block's successors have a single predecessor (though that may be the case currently; otherwise should probably not be considered atm). Supplying `removeOnEdge(Block)` which disconnects Block from its single Pred and single Succ, reconnecting them together while retaining pred and succ positions may help, but an empty block being merged into its predecessor may have multiple successors - rather than being on an edge.

https://github.com/llvm/llvm-project/pull/137709