Minor tweak for better StateVector error messages

If you use StateVector<Block*,...> or similar on accident,
you get a bunch of junk.  Add a static_assert so there's at least a
reasonable error in there somewhere.

hphp/runtime/vm/translator/hopt/state_vector.h

@@ -17,6 +17,8 @@
 #ifndef incl_HPHP_JIT_STATE_VECTOR_H_
 #define incl_HPHP_JIT_STATE_VECTOR_H_
 
+#include <type_traits>
+
 #include "hphp/runtime/base/memory/memory_manager.h"
 #include "hphp/runtime/vm/translator/hopt/irfactory.h"
 
@@ -39,6 +41,13 @@ struct StateVector {
   typedef typename InfoVector::reference reference;
   typedef typename InfoVector::const_reference const_reference;
 
+  static_assert(
+    std::is_same<Key,Block>::value ||
+    std::is_same<Key,IRInstruction>::value ||
+    std::is_same<Key,SSATmp>::value,
+    "StateVector can only be used with Block, IRInstruction, or SSATmp"
+  );
+
   StateVector(const IRFactory* factory, Info init)
     : m_factory(factory)
     , m_info(numIds(factory, static_cast<Key*>(nullptr)), init)