Insert a new intrinsic call after splitting critical edges, and verify
it. Later commits will update the SSA values to use this new value along
indirect branches rather than the callbr's value, and have SelectionDAG
consume this new value.
Part 2b of
https://discourse.llvm.org/t/rfc-syncing-asm-goto-with-outputs-with-gcc/65453/8.
I'm not sure we need to document this at all (it's an unstable implementation detail users of LLVM shouldn't need to be aware of). But if you do think it's necessary to document it, please call out that it's an unstable implementation detail, and users should not call it.