Refactor Hi and Lo out of Variable.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1326013002 .
diff --git a/src/IceCfg.h b/src/IceCfg.h
index 62a8914..6a1a1c8 100644
--- a/src/IceCfg.h
+++ b/src/IceCfg.h
@@ -290,6 +290,8 @@
   static void TlsInit() { ICE_TLS_INIT_FIELD(CurrentCfg); }
 };
 
+template <> Variable *Cfg::makeVariable<Variable>(Type Ty);
+
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICECFG_H