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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

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).

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