[turbofan] Exhaustive switches for MachineRepresentation.

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index e0407527fa66d52d8cb5f8ad64bd728d4c0b6e33..80fdb135e38a33d5b2e8de0ee5c7d61689cd61e7 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -104,10 +104,11 @@ int GetByteWidth(MachineRepresentation rep) {
     case MachineRepresentation::kWord64:
     case MachineRepresentation::kFloat64:
       return 8;
-    default:
-      UNREACHABLE();
-      return 0;
+    case MachineRepresentation::kNone:
+      break;
   }
+  UNREACHABLE();
+  return 0;
 }
 
 }  // namespace