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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Hi - yeah I was expecting Root to always be replaced, hence it should be correct either way. Not modifying Root feels a little cleaner though. You would not usually expect the old code to be modified as the new nodes are created, and it can be a bit confusing to follow the logic of setting the on the Root, to be used later on (in code that is easy to miss), to override the flags of the instructions that have just been created.

> DelInstrs.push_back(&Root); 

Yeah I was thinking that would need to be added, either to the switch or into the genSubAdd2SubSub method.

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