[Inline] Fix a warning by removing an explicit copy constructor

This patches fixes the warning:

  llvm/include/llvm/Analysis/InlineCost.h:62:3: error: definition of
  implicit copy assignment operator for 'CostBenefitPair' is
  deprecated because it has a user-declared copy constructor
  [-Werror,-Wdeprecated-copy]

by removing the explicit copy constructor.