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/memflags.rs

@@ -14,7 +14,7 @@ const NAMES: [&str; 2] = ["notrap", "aligned"];
 /// Each of these flags introduce a limited form of undefined behavior. The flags each enable
 /// certain optimizations that need to make additional assumptions. Generally, the semantics of a
 /// program does not change when a flag is removed, but adding a flag will.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub struct MemFlags {
     bits: u8,
 }