Loop analysis of the IL

* Implemented in two passes
* First pass discovers the loops headers (they dominate one of their predecessors)
* Second pass traverses the blocks of each loop
* Discovers the loop tree structure
* Offers a new LoopAnalysis data structure queried from outside the module

lib/cretonne/src/entity_list.rs

@@ -94,7 +94,7 @@ impl<T: EntityRef> PartialEq for EntityList<T> {
 impl<T: EntityRef> Eq for EntityList<T> {}
 
 /// A memory pool for storing lists of `T`.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct ListPool<T: EntityRef> {
     // The main array containing the lists.
     data: Vec<T>,