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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Should this be enabled or not at optsize/minsize (as in instr-size-costs.ll)?
And should it be disabled for all the cases where it is not expected to be profitable? For any in-order core for example I believe this will just add extra instructions for likely no extra benefit? I may be mistaken about what exactly it does.

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