Incorporate iter_nodes into graph::DirectedGraph

This assumes that the set of valid node IDs is exactly `0..num_nodes`.

In practice, we have a lot of graph-algorithm code that already assumes that
nodes are densely numbered, by using `num_nodes` to allocate per-node indexed
data structures.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -10,14 +10,12 @@ use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
 
 use crate::coverage::counters::balanced_flow::BalancedFlowGraph;
-use crate::coverage::counters::iter_nodes::IterNodes;
 use crate::coverage::counters::node_flow::{
     CounterTerm, NodeCounters, make_node_counters, node_flow_data_for_balanced_graph,
 };
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph};
 
 mod balanced_flow;
-mod iter_nodes;
 mod node_flow;
 mod union_find;