Handle index expressions' callee IDs correctly

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.

src/libsyntax/ast_util.rs

@@ -455,7 +455,7 @@ fn id_visitor(vfn: fn@(node_id)) -> visit::vt<()> {
         visit_expr: fn@(e: @expr) {
             vfn(e.id);
             alt e.node {
-              expr_unary(_, _) | expr_binary(_, _, _) {
+              expr_unary(*) | expr_binary(*) | expr_index(*) {
                 vfn(ast_util::op_expr_callee_id(e));
               }
               _ { /* fallthrough */ }