[llvm] [LoopUnroll] Introduce parallel reduction phis when unrolling. (PR #149470)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I was imprecise when I said you "can't unroll" that loop; technically you can unroll almost anything.  But your current version of reduction detection won't work for that loop, and making reduction detection work would require significant infrastructure changes to unrolling, to introduce runtime checks.

https://github.com/llvm/llvm-project/pull/149470