Switch isl back to isl 0.12.1

The newer isl version broke backward compatibility. For some reason, I did not
find this in my own tests.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@199586 91177308-0d34-0410-b5e6-96231b3b80d8
Adjust test case to changed cloog output

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@199587 91177308-0d34-0410-b5e6-96231b3b80d8