Test that independent block pass does not transform induction variables

The original test case showed a problem with the independet blocks pass and
we decided to XFAIL it for now. Unfortunately the failure is not detected if
we build without asserts and the verification of the independent block pass
is not run. This change tests now for the actual reason of the failure and
should trigger even in a non asserts build. We did not yet solve the underlying
bug, but this should at least make the test suite behavior consistent.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@183025 91177308-0d34-0410-b5e6-96231b3b80d8