[Fix] Allow pointer types as access elements and compare them correctly

  This fixes two problems which are usualy caused together:
    1) The elements of an isl AST access expression could be pointers
       not only integers, floats and vectores thereof.
    2) The runtime alias checks need to compare pointers but if they
       are of a different type we need to cast them into a "max" type
       similar to the non pointer case.