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/lib.rs

@@ -20,6 +20,7 @@ pub mod entity_map;
 pub mod flowgraph;
 pub mod ir;
 pub mod isa;
+pub mod loop_analysis;
 pub mod regalloc;
 pub mod result;
 pub mod settings;