Member predicate Node::asIndirectArgument

Gets the the argument going into a function for a node that represents the indirect value of the argument after index loads. For example, in:

void f(int**);
...
int** x = ...;
f(x);

The node n such that n.asIndirectArgument(1) represents the value of *x going into f, and the node n such that n.asIndirectArgument(2) represents the value of **x going into f.