It’s not that IR setjump is useful in itself, more that compiling most of libc to IR and then having to put that in an archive to bundle a couple of object files with it as well is a nuisance. If it’s all wrapped in IR, that single .bc file can be shipped around as-is.
IR libraries are a thing already, if slightly contentious. If we ultimately fix the long tail of libc assumptions in llvm, it’s the right format for statically linking executables with link time optimisation.