[MachinePipeliner] Fix elements being added while the list is iterated (#80805)

There is no need to add the elements of Objs twice, so the addition is
removed.

Author: ytmukai

llvm/lib/CodeGen/MachinePipeliner.cpp

@@ -768,7 +768,6 @@ static void getUnderlyingObjects(const MachineInstr *MI,
       Objs.clear();
       return;
     }
-    Objs.push_back(V);
   }
 }