[llvm] [SLP]Initial non-power-of-2 support (but still whole register) for reductions (PR #112361)

[Nikita Popov via llvm-commits]

nikic wrote:

It looks like this causes some compile-time regressions: https://llvm-compile-time-tracker.com/compare.php?from=ec78f0da0e9b1b8e2b2323e434ea742e272dd913&to=8287fa8e596d8fc8655c8df3bc99e068ad9f7d4b&stat=instructions:u

For example lencod me_distortion.c regresses by 14% without code changes.

https://github.com/llvm/llvm-project/pull/112361