[Metadata] Fix `addAnnotationMetadata` when appending a string tuple to an existing MDTuple.

[zjaffal]

Summary

Currently if the MD_Annotation node containd a string node when we add a
string tuple node the string will not be preserved. This change fixes
that issue.

[fhahn]

Is it possible to have a test for this?

[zjaffal]

In D152448#4406179, @fhahn wrote:

Is it possible to have a test for this?

At the moment we cannot add a test. Unless we create a dummy pass and call the function between the passes and verify if the metadata gets appended correctly

[fhahn]

Ah makes sense, thanks! LGTM

llvm/lib/IR/Metadata.cpp

SmallSetVector<StringRef, 2> AnnotationsSet(Annotations.begin(),
                                               Annotations.end());
   MDBuilder MDB(getContext());
 
   auto *Existing = getMetadata(LLVMContext::MD_annotation);
   SmallVector<Metadata *, 4> Names;
   if (Existing) {
     auto *Tuple = cast<MDTuple>(Existing);
     for (auto &N : Tuple->operands()) {
-      if (isa<MDString>(N.get()))
+      if (isa<MDString>(N.get())) {
+        Names.push_back(N);
         continue;
+      }
       auto *MDAnnotationTuple = cast<MDTuple>(N);
       if (any_of(MDAnnotationTuple->operands(), [&AnnotationsSet](auto &Op) {
             return AnnotationsSet.contains(cast<MDString>(Op)->getString());
           }))
         return;
       Names.push_back(N);
     }
   }