Create a dependence struct to hold dependence information for a SCoP.

  The new Dependences struct in the DependenceInfo holds all information
  that was formerly part of the DependenceInfo. It also provides the
  same interface for the user to access this information.

  This is another step to a more general ScopPass interface that does
  allow multiple SCoPs to be "in flight".

llvm-svn: 231327