Update to the latest version of isl

Isl now specifically marks modulo operations that are compared against zero.
They can be implemented with the C/LLVM remainder operation.

We also update a couple of test cases where the output of isl has slightly
changed.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223607 91177308-0d34-0410-b5e6-96231b3b80d8
Drop unused 'docs' directory

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223608 91177308-0d34-0410-b5e6-96231b3b80d8