Fix the modifiable access creation

  + Remove the class IslGenerator which duplicates the functionality of
    IslExprBuilder.
  + Use the IslExprBuilder to create code for memory access relations.
    + Also handle array types during access creation.
  + Enable scev codegen for one of the transformed memory access tests,
    thus access creation without canonical induction variables available.
  + Update one test case to the new output.