Revert "Revert "[LoopNest] Fix Wdeprecated-copy warnings""

This reverts commit 07ef5805ab.

The broke of the sanitizer-windows bot:
https://lab.llvm.org/buildbot/#/builders/127/builds/12064
is not caused by the original commit.

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

llvm/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);