Optimizer can't remove unused code without side effects

[Erik_Eckstein]

@dmt

An instance of a String-specialized generic class can't be eliminated, but an instance of an Int-specialized one can.

Unfortunately this needs some more work in the compiler. It requires that

1. DeadObjectElimination supports OSSA
2. We further propagate OSSA down the optimizer pipeline - after the first inlining pass.

Do you mind filing another github issue for this?