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

[Nicholas Guy via llvm-commits]

NickGuy-Arm wrote:

As a side effect of this work being motivated by supporting complex dot product instructions (and thus allowing use of straight loads instead of gathers/deinterleaving loads), this PR also introduces support for partial reduction subtractions via negating the second operand of `partial.reduce.add` calls.

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