[llvm] [LoopVectorizer] Add support for chaining partial reductions (PR #120272)

[Nicholas Guy via llvm-commits]

NickGuy-Arm wrote:

> Sounds like supporting partial reduction subtractions would be a good extension on its own (and test-able independently), could it be split off?

Yep. I've pulled it out completely, and will create a new PR for adding sub support.

https://github.com/llvm/llvm-project/pull/120272