isl scheduler: Do not fail when returning an empty band list

The bug was within isl. To fix it, we simply update the isl version that
is used by Polly. We still have some changes within Polly to be able to
write a proper test case.

Reported-by: Sameer Sahasrabuddhe <Sameer.Sahasrabuddhe@amd.com>
llvm-svn: 166021