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.

cranelift/src/filetest/mod.rs

@@ -20,6 +20,7 @@ mod legalizer;
 mod regalloc;
 mod runner;
 mod runone;
+mod simple_gvn;
 mod verifier;
 
 /// The result of running the test in a file.
@@ -62,6 +63,7 @@ fn new_subtest(parsed: &TestCommand) -> subtest::Result<Box<subtest::SubTest>> {
         "legalizer" => legalizer::subtest(parsed),
         "regalloc" => regalloc::subtest(parsed),
         "binemit" => binemit::subtest(parsed),
+        "simple-gvn" => simple_gvn::subtest(parsed),
         _ => Err(format!("unknown test command '{}'", parsed.command)),
     }
 }