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.

llvm-svn: 226520

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -572,7 +572,7 @@ void BitcodeReaderMDValueList::tryToResolveCycles() {
 
   // Resolve any cycles.
   for (auto &MD : MDValuePtrs) {
-    auto *N = dyn_cast_or_null<UniquableMDNode>(MD);
+    auto *N = dyn_cast_or_null<MDNode>(MD);
     if (!N)
       continue;