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

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

> Curious if/how this will interact with #136173?
> 
> Would it potentially help to simplify computeCost & co?

I think we'll want to turn a partial reduction without a bin op into a `VPExtendedReductionRecipe`  and then lower it back to a partial reduction.

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