[turbofan] Make MachineType a pair of enums.

src/compiler/diamond.h

Index: src/compiler/diamond.h
diff --git a/src/compiler/diamond.h b/src/compiler/diamond.h
index f562092a8a4395f7acadb89df68dc84e0a2d4882..e1333052d745bbf12769781283dec8ed96eb2ae4 100644
--- a/src/compiler/diamond.h
+++ b/src/compiler/diamond.h
@@ -49,8 +49,8 @@ struct Diamond {
     }
   }
 
-  Node* Phi(MachineType machine_type, Node* tv, Node* fv) {
-    return graph->NewNode(common->Phi(machine_type, 2), tv, fv, merge);
+  Node* Phi(MachineRepresentation rep, Node* tv, Node* fv) {
+    return graph->NewNode(common->Phi(rep, 2), tv, fv, merge);
   }
 };