Use stringFromIslObj instead of isl_..._dump to print to dbgs()

This makes sure we consistently use dbgs() when printing debug output.
Previously, the code just mixed calls to isl_*_dump() with printing to dbgs()
and was relying for both methods to interact in predictable ways (same output
stream, no unexpected reordering of outputs).

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@220443 91177308-0d34-0410-b5e6-96231b3b80d8