dead code detection

[aviatesk]

There are bunch of dead code around (like JuliaLang/julia#42833), and it would be nice if JET can report such cases automatically.

Especially, array allocation couldn't be eliminated by the compiler even if there is no "usage" of the array,
but usually those allocations are just dead code, so a static analysis would be very useful to eliminate such unnecessary computations.

[dpinol]

👍 I currently detect some cases on my code with a sloppy multiline grep, but it would be cool if jet could make it