[llvm] [LoopVectorize] Add support for vectorisation of more early exit loops (PR #88385)

[David Sherwood via llvm-commits]

david-arm wrote:

* Tidied up requiresScalarEpilogue and fixupIVUsers
* Added the original early exit block to State.CFG as a more appropriate place than the VPRegionBlock.
* Cleaned up the createInitialVPlan interface so now we only pass in a boolean indicating whether we have an early exit or not.
* Updated VPLiveOut::fixPhi to use the new incoming block parameter instead of trying to reverse engineer the CFG.

https://github.com/llvm/llvm-project/pull/88385