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/lib.rs

@@ -32,6 +32,7 @@ mod constant_hash;
 mod context;
 mod iterators;
 mod legalizer;
+mod licm;
 mod packed_option;
 mod partition_slice;
 mod predicates;