[LoopNest] Fix Wdeprecated-copy warnings

error: definition of implicit copy constructor for 'LoopNest' is
deprecated because it has a user-declared copy assignment operator
[-Werror,-Wdeprecated-copy]
  LoopNest &operator=(const LoopNest &) = delete;

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D103752

include/llvm/Analysis/LoopNestAnalysis.h

@@ -30,7 +30,6 @@ public:
   LoopNest(Loop &Root, ScalarEvolution &SE);
 
   LoopNest() = delete;
-  LoopNest &operator=(const LoopNest &) = delete;
 
   /// Construct a LoopNest object.
   static std::unique_ptr<LoopNest> getLoopNest(Loop &Root, ScalarEvolution &SE);