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.

filetests/simple_gvn/basic.cton

@@ -0,0 +1,11 @@
+test simple-gvn
+
+function simple_redundancy(i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32):
+    v2 = iadd v0, v1
+    v3 = iadd v0, v1
+; check: v3 -> v2
+    v4 = imul v2, v3
+; check: v4 = imul $v2, $v3
+    return v4
+}