Stop relying on MDNode::get() returning self-references

Update for LLVM API change, wherein self-references are not uniqued (and
will never be returned by `MDNode::get()`).  We already have `Id`; just
return it.

This should fix the failing buildbot:

  http://lab.llvm.org:8011/builders/polly-amd64-linux/builds/25167
  http://lab.llvm.org:8011/builders/polly-amd64-linux/builds/25168

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223620 91177308-0d34-0410-b5e6-96231b3b80d8