Update isl to latest maintenance release

This includes isl commits b9c7b8c8c98a6722262f076b27f833a25b83f55d and
454621e935ff836361c77df436fbd7066c9d44e0 which fix PR18130.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@196655 91177308-0d34-0410-b5e6-96231b3b80d8