[llvm] [LoopVectorize] Support vectorization of compressing patterns (PR #214491)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> It looks like this allows outside users of the conditional IV phi, but doesn't update the live outs, so it will return the wrong final value for e.g:

To keep things simple for now I've updated the legality checks to only allow in-loop users derived from the phi (that are used to compute the addresses of loads/stores). This means only the backedge value can be used as a live out.

https://github.com/llvm/llvm-project/pull/214491