+/* Walks over the ir graph, starting at the node given as first argument.
+ Executes pre before visiting the predecessor of a node, post after.
+ irg_walk uses the visited flag in irg and the nodes to determine visited
+ nodes. It executes inc_irg_visited(current_ir_graph) to generate a new
+ flag. It marks the node as visited before executing pre.
+ The void* env can be used to pass status information between the
+ pre and post functions. */