I don't see why a new compiler would be necessary. It's unlikely they'd completely throw out the whole ARM ISA and replace it with something completely different in one go. More likely, they'll gradually add new instructions and mutate the characteristics of existing instructions over time. LLVM IR isn't completely ISA agnostic, a lot of ISA specific assumptions are still baked into it, but gradual iterative improvement is Apple's speciality. A couple of new instructions this year, deprecate a few old one next, and in 5 years time you've got a new ISA that's barely recognizably ARM any more yet each step along the way is just an incremental and largely backwards compatible change.