[llvm] [LoopUnroll] Introduce parallel accumulators when unrolling reductions with vector instructions. (PR #166353)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, thanks.

Would probably be good to adjust the title to be clear that this adjust IVDescriptors to support detecting vector reductions.

This should be fine; other users already guard against vector instructions separately.

https://github.com/llvm/llvm-project/pull/166353