Allow using `ReturnNode()` in `@generated` code (#51715)
IRTools.jl currently tries to use `ReturnNode()` to model unreachable
block terminators but it fails in `find_ssavalue_uses`. This PR adds a
check to enable using `ReturnNode()` in untyped code.
One other alternative for frontends is to use an instruction known to
terminate (`Core.throw`) instead. See
https://github.com/FluxML/IRTools.jl/pull/115 for more context.
