s2wasm: Do not add drops for void values

[dschuff]

Fixes #708

[kripken]

In #708 we also saw that the type of exit looks wrong (function type is void, but it has a return statement with a value) - is that an unrelated problem to the one this PR fixes?

Separately, can we enable drop validation with this PR? That is, replace the code in src/wasm-validator.h for visitDrop(), which currently checks an env var, with something like

void visitDrop(Drop* curr) {
  shouldBeTrue(isConcreteWasmType(curr->value->type) || curr->value->type == unreachable, curr, "can only drop a valid value");
}

[dschuff]

I think the exit type being wrong is a problem with the .s file and not with s2wasm. Also there's one further failure in the wasm-opt tests before we can turn on drop validation:

.. vacuum.wast
executing:  bin/wasm-opt --vacuum split.wast --print
[wasm-validator error in function $if-drop] unexpected false: Dropping a none value, on 
(drop
  (if
    (call $if-drop)
    (call $int)
    (br $out)
  )
)
[wasm-validator error in function $if-drop] unexpected false: Dropping a none value, on 
(drop
  (if
    (call $if-drop)
    (br $out)
    (call $int)
  )
)
Fatal: error in validating input

[kripken]

Ok, thanks, lgtm. I'll investigate that drop validation problem.