Remove assignment warnings generated when calling void functions.

[Fleli]

Simply calling void functions using the call keyword, e.g.

int main() {
    call f();
    return 0;
}

void f() {
    // ...
}

yields a warning such as

Assignment to '%1' of type 'void' has no effect.

This is because all function call returns are assigned to a variable under the hood. But since f returns void, a warning is generated (the compiler sees the call as void %1 = f();).

Remove this warning.