Allow multidimensional accesses in the IslExprBuilder.

  This resolved the issues with delinearized accesses that might alias,
  thus delinearization doesn't deactivate runtime alias checks anymore.

Differential Revision: http://reviews.llvm.org/D5614


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@219078 91177308-0d34-0410-b5e6-96231b3b80d8