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

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

At a high level, do we really want to do this in the unroller?  We can already "unroll" loops like this, if you pass `#pragma clang loop vectorize_width(1) interleave_count(4)`, and the vectorizer has better infrastructure for cost modeling.

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