[llvm] [AArch64] Drop poison-generating flags in `genSubAdd2SubSub` combiner (PR #90028)

[via llvm-commits]

v01dXYZ wrote:

@davemgreen Out of curiosity, you seem to prefer not to not mutate `Root` if possible.

Is `Root` always replaced ? (it doesn't seem so but I prefer to ask).

https://github.com/llvm/llvm-project/pull/90028