This is operating on IR, not on lines of code. Figuring out where the operation came from is extremely difficult because you have to propagate all of that info back and forth across the passes, and it may end up being split "across" syntactic elements. If your language has any form of metaprogramming or code reuse (i.e. all of them), that operation may also be necessary at some use sites and not at others, among other issues.

This is not always possible. Consider the monomorphized output of a generic function. An operation may be dead in one instance but not generally

This kind of compiler/runtime feedback about source code is really interesting and (imo) under-studied. Especially when you take into account something like PGO data.