Start a very simple GVN pass (#79)

* Skeleton simple_gvn pass.
* Basic testing infrastructure for simple-gvn.
* Add can_load and can_store flags to instructions.
* Move the replace_values function into the DataFlowGraph.
* Make InstructionData derive from Hash, PartialEq, and Eq.
* Make EntityList's hash and eq functions panic.
* Change Ieee32 and Ieee64 to store u32 and u64, respectively.

lib/cretonne/src/lib.rs

@@ -35,5 +35,6 @@ mod packed_option;
 mod partition_slice;
 mod predicates;
 mod ref_slice;
+mod simple_gvn;
 mod topo_order;
 mod write;