It is a pointer to Value. Try cast<>ing it to ConstantInt and then calling getValue():

for(auto arg = fn->arg_begin(); arg != fn->arg_end(); ++arg) {
  if(auto* ci = dyn_cast<ConstantInt>(arg))
    errs() << ci->getValue() << "\n";
  errs() << *arg << "\n";
}

You can use Function::getArgumentList() method to get a list of function's arguments. Then, you traverse it with iterators - ArgumentListType::begin() and ArgumentListType::end().

See class Function documentation - http://llvm.org/doxygen/classllvm_1_1Function.html

UPD:

The current way of iterating over arguments is arg_begin()/arg_end()/args() methods.

Adding arguments to a function is surprisingly tricky - as you've discovered, it's not as easy as modifying the argument list.

The most foolproof way of doing it is probably to first create a new function with all the original arguments + the extra arguments, and then call CloneFunctionInto to embed the original function inside your new function.

You should not load anything. Use Function::arg_iterator to get the Value's corresponding to the arguments.

See http://llvm.org/docs/doxygen/html/classllvm_1_1Function.html (arg_begin / arg_end) and http://llvm.org/docs/ProgrammersManual.html#Function for more information