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

[via llvm-commits]

mamasten wrote:

We've run into a performance regression and would like to have a choice to disable the feature to see how it impacts other workloads. I'm gathering more information on the regression and can probably provide a test case in the near future.

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