I can't say how LLVM does it but in a compiler I built I detected tail recursion by scanning the IR and if the value of a recursive call (ie: call to self) was

I can't say how LLVM does it but in a compiler I built I detected tail recursion by scanning the IR and if the value of a recursive call (ie: call to self) was immediately returned it was considered a candidate for TCE. If the value was consumed by the parent, it wasn't.