Fix typo

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@222559 91177308-0d34-0410-b5e6-96231b3b80d8
Use isl_schedule_get_ctx

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@222560 91177308-0d34-0410-b5e6-96231b3b80d8