[Refactor][NfC] Simplify and clean the handling of (new) access relations

  This patch does not change the semantic on it's own. However, the
  dependence analysis as well as dce will now use the newest available
  access relation for each memory access, thus if at some point the json
  importer or any other pass will run before those two and set a new
  access relation the behaviour will be different. In general it is
  unclear if the dependence analysis and dce should be run on the old or
  new access functions anyway. If we need to access the original access
  function from the outside later, we can expose the getter again.

Differential Revision: http://reviews.llvm.org/D5707