This has been stated before: LLVM Bitcode is architecture specific. It's dependent on the architectures ABI and can contain inline assembly. There are LLVM targets with corresponding ABIs that are nonspecific to architectures like PNaCL but Apple is not using them
Things Apple can do with bitcode:
Produce binaries optimized for different ARM processors. Reoptimize with new versions of LLVM as soon as they are available. Use a big database of LLVM IR to tweak optimizations. Keep some optimizations secret and not even publish binaries doing them.
The biggest argument IMO that speaks against an Apple ISA is that they would have to rewrite tons of hand tuned assembly code.
Things Apple can do with bitcode: Produce binaries optimized for different ARM processors. Reoptimize with new versions of LLVM as soon as they are available. Use a big database of LLVM IR to tweak optimizations. Keep some optimizations secret and not even publish binaries doing them.
The biggest argument IMO that speaks against an Apple ISA is that they would have to rewrite tons of hand tuned assembly code.