LICM pass (#87)

* LICM pass

* Uses loop analysis to detect loop tree
* For each loop (starting with the inner ones), create a pre-header and move there loop-invariant instructions
* An instruction is loop invariant if it does not use as argument a value defined earlier in the loop
* File tests to check LICM's correctness
* Optimized pre-header creation
If the loop already has a natural pre-header, we use it instead of creating a new one.
The natural pre-header of a loop is the only predecessor of the header it doesn't dominate.

lib/cretonne/src/context.rs

@@ -19,6 +19,7 @@ use regalloc;
 use result::CtonResult;
 use verifier;
 use simple_gvn::do_simple_gvn;
+use licm::do_licm;
 
 /// Persistent data structures and compilation pipeline.
 pub struct Context {
@@ -92,6 +93,15 @@ impl Context {
         self.verify(None).map_err(Into::into)
     }
 
+    /// Perform LICM on the function.
+    pub fn licm(&mut self) -> CtonResult {
+        do_licm(&mut self.func,
+                &mut self.cfg,
+                &mut self.domtree,
+                &mut self.loop_analysis);
+        self.verify(None).map_err(Into::into)
+    }
+
     /// Run the register allocator.
     pub fn regalloc(&mut self, isa: &TargetIsa) -> CtonResult {
         self.regalloc