[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