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/ir/instructions.rs

@@ -98,7 +98,7 @@ impl FromStr for Opcode {
 /// value should have its `ty` field set to `VOID`. The size of `InstructionData` should be kept at
 /// 16 bytes on 64-bit architectures. If more space is needed to represent an instruction, use a
 /// `Box<AuxData>` to store the additional information out of line.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
 #[allow(missing_docs)]
 pub enum InstructionData {
     Nullary { opcode: Opcode },