[llvm] [VPlan] Use ResumePhi to create reduction resume phis. (PR #110004)

[via llvm-commits]

https://github.com/ayalz commented:

Would be good to clarify various related aspects, such as where ResumePhi's already appear, if this patch is the first to populate the scalar pre-header with recipes, why handling the reduction of epilog loop requires special attention.

https://github.com/llvm/llvm-project/pull/110004