[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.
> > Edit; PR for adding subtraction partial reductions: #123636
> 
> Ok great thanks! It’s a deft at the moment, any issues?

Only when isolating/creating the tests. As far as I can tell, subtraction partial reductions will rarely, if ever, be provided as an input to vplan (due to no `RecurKind::Sub`), I haven't looked into it since Monday though, so maybe there's a simpler test case I haven't found yet.
That, and partial-reduce-chained.ll from this PR will need to be updated too..

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