[SystemZ] Make sure SelectCode() is not called on a target opcode.

Since getNode() might not always return the requsted opcode, for instance if
called with (ISD::AND, -1) arguments, there should be a check so that
SelectCode() is only called when appropriate.

Review: Ulrich Weigand
llvm-svn: 326178

llvm/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp

@@ -993,7 +993,8 @@ bool SystemZDAGToDAGISel::tryRISBGZero(SDNode *N) {
         N = New.getNode();
       }
       // Now, select the machine opcode to implement this operation.
-      SelectCode(N);
+      if (!N->isMachineOpcode())
+        SelectCode(N);
       return true;
     }
   }