Some code that handles unary and binary exprs' callee IDs was
forgetting to handle the index expr case (since calls to
user-defined index operators also have callee IDs). This was
manifesting as an ICE in trans because when monomorphizing a
function that had one of these operators in it (an index into a
dvec, in the test case), the callee ID would be unbound to a type.
Fixed it. Closes#2631.
