[CodeGen][NFC] Declare copy constructor & copy assignment as deleted for ScheduleDAG

[skan]

Summary

ScheduleDAG has derived classes ScheduleDAGVLIW and ScheduleDAGRRList,
which own resources that are freed in their destructors. Static analyzer
warns b/c they do not have user-written copy constructors.

According to the design of ScheduleDAG, it seems that it should always
be passed by reference. So I declare them as deleted in this patch.

[RKSimon]

LGTM with one minor

llvm/include/llvm/CodeGen/ScheduleDAG.h

571

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[skan]

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llvm/include/llvm/CodeGen/ScheduleDAG.h

public:
     SUnit ExitSU;                       ///< Special node for the region exit.
 
 #ifdef NDEBUG
     static const bool StressSched = false;
 #else
     bool StressSched;
 #endif
 
+    // This class is designed to be passed by reference only. Copy constructor
+    // is declared as deleted here to make the derived classes have deleted
+    // implicit-declared copy constructor, which suppresses the warnings from
+    // static analyzer when the derived classes own resources that are freed in
+    // their destructors, but don't have user-written copy constructors (rule
+    // of three).
+    ScheduleDAG(const ScheduleDAG &) = delete;
+    ScheduleDAG &operator=(const ScheduleDAG &) = delete;
+
     explicit ScheduleDAG(MachineFunction &mf);
 
     virtual ~ScheduleDAG();
 
     /// Clears the DAG state (between regions).
     void clearDAG();
 
     /// Returns the MCInstrDesc of this SUnit.