Warn about unreachable code.

[ekpyron]

Closes #2340.

This still does not consider constant conditions (e.g. if (false) ...). I would only add support for that, once we have a full solidity-level compile-time constant expression evaluator, though (which at least I would say we should have).

[chriseth]

I guess this logic does not detect that e.g. a function always reverts, and thus calls to that function will have similar semantics to revert(), right?
We might want to add that (in a subsequent feature) so that the warning is as powerful for code that implements helper functions for revert.

[chriseth]

Another question to discuss: Do we want to make this an error?

[ekpyron]

I think this is too fragile to make an error (because of the if (false)-like cases... detection won't be perfect in all cases). Actually those are all false negatives... hm... I have to think about it.

Function calls are basically ignored in the CFG so far, but yes, we can change that in the future (out of scope for now I would say, though).

[ekpyron]

I think we could make this an error, if we want to. But to me it seems a warning would suffice... we could remove the dead code ourselves during optimization.

[axic]

Nice!

[ekpyron]

Should be squashed before merging.

[chriseth]

Fine apart from the small changes. Also, should this be squashed to a single commit?

Tried to address the small changes and squashed to one commit.