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

[Graham Hunter via llvm-commits]

https://github.com/huntergr-arm commented:

I think it should be possible to split out the VPlan changes and test via llvm/unittests/Transforms/Vectorize/VPlanTest.cpp (and the other files in that directory).

I've experimented with this implementation a bit, and it doesn't quite model a single-entry-multiple-exit region. The vector early exit block seems like it's part of the vector region in the plan, when it should be external to it as it's not part of the loop. Separating out the vplan changes should let us see this and figure out how to represent the CFG properly.

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