[CodeGen] Disable default copy ctor and copy assignment operator for PressureDiffs

[yubing]

ping?

llvm/include/llvm/CodeGen/RegisterPressure.h

 /// Array of PressureDiffs.
 class PressureDiffs {
   PressureDiff *PDiffArray = nullptr;
   unsigned Size = 0;
   unsigned Max = 0;
 
 public:
   PressureDiffs() = default;
+  PressureDiffs &operator=(const PressureDiffs &other) = delete;
+  PressureDiffs(const PressureDiffs &other) = delete;
   ~PressureDiffs() { free(PDiffArray); }
 
   void clear() { Size = 0; }
 
   void init(unsigned N);
 
   PressureDiff &operator[](unsigned Idx) {
     assert(Idx < Size && "PressureDiff index out of bounds");