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

[Florian Hahn via llvm-commits]

https://github.com/fhahn 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.

Thanks, updated the description. This follows https://github.com/llvm/llvm-project/commit/9a5a8731e77bb61c64d400a828dc215dc80fb839 which added ResumePHIs for FOR resume values to the scalar preheader. Epilogue needs special attention, as it needs extra incoming values from the bypass blocks.

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