Use isl::manage_copy to simplify calls to isl::manage(isl_.._copy())

As part of this cleanup a couple of unnecessary isl::manage(obj.copy()) pattern
are eliminated as well.

We checked for all potential cleanups by scanning for:

  "grep -R isl::manage\( lib/ | grep copy"

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@325558 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed