version_69: Dead return value elimination in DeadArgumentElimination (#1917)

* Finds functions whose return value is always dropped, and removes the return.
* Run multiple iterations of the pass, as one can enable others.
* Do not run DeadArgumentElimination at all if debug info is present (with these improvements, it became much more likely to destroy debug info).

Saves 2.5% on hello world, because of some simple libc calls.