[llvm] [AArch64][MachineCombiner] Reassociate long chains of accumulation instructions into a tree to increase ILP (PR #126060)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

I'm not familiar with AArch64 target so I added some reviewers and changed the title to increase visibility.

https://github.com/llvm/llvm-project/pull/126060