IR: Merge UniquableMDNode back into MDNode, NFC

As pointed out in r226501, the distinction between `MDNode` and
`UniquableMDNode` is confusing.  When we need subclasses of `MDNode`
that don't use all its functionality it might make sense to break it
apart again, but until then this makes the code clearer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226520 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/DIBuilder.cpp

@@ -105,8 +105,8 @@ void DIBuilder::finalize() {
   // Now that all temp nodes have been replaced or deleted, resolve remaining
   // cycles.
   for (const auto &N : UnresolvedNodes)
-    if (N)
-      cast<UniquableMDNode>(N)->resolveCycles();
+    if (N && !N->isResolved())
+      N->resolveCycles();
   UnresolvedNodes.clear();
 
   // Can't handle unresolved nodes anymore.