[llvm] [LV] Create in-loop sub reductions (PR #147026)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm approved this pull request.

LGTM with nit addressed.

@davemgreen wrote:
> Hi - Could this add a sub into the middle of an extending reduction?
> 
> (It might be good to transform a sub reduction into a add reduction + negate at the end of the loop, to allow the more efficient sum reductions to trigger).

That's not a bad idea, although that might be better left for a future PR. The reason for this change is to align the implementations of partial and in-loop reductions. I'm also not entirely sure if we necessarily want to do this for partial reductions, because some of the `cdot` patterns rely on the input being negated inside the loop.

https://github.com/llvm/llvm-project/pull/147026