Update to isl-0.13.0

This pulls in a couple of minor cleanups in isl. More importantly, in
preparation of the upcoming LLVM releases this change brings us back on a
released version of isl.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@213062 91177308-0d34-0410-b5e6-96231b3b80d8