[llvm] [SelectionDAG] Use computeForAddSub to prove that ISD::ADD is not 0 (PR #86453)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@AtariDreams Its great that you're working through the (far too many.......) TODOs in the codebase, but you have to begin with creating tests that show the missing functionality, not just add code and hope it gets tested.

https://github.com/llvm/llvm-project/pull/86453