CodeGeneration: Fix double free in vector for

We now use __isl_take to annotate the uses of the isl_set where we got the
memory management wrong.

Thanks to Rafael! His pipefail work hardened our test environment and exposed
this bug nicely.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@187338 91177308-0d34-0410-b5e6-96231b3b80d8