[llvm] [LV] Add support for partial reductions without a binary op (PR #133922)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

Hi @MDevereau the code that this PR touches is currently being refactored quite significantly. I think it's worth holding off a little longer until #144908 and #144281 make it in, and then rebase this patch on top of those changes. Those patches should also make it easier to model this improvement.

https://github.com/llvm/llvm-project/pull/133922