updated some comments.

there is no code change, just added some comments on why code bail out and use full semantic model rather than speculative semantic model in some situation. (changeset 1408980)

src/Workspaces/Core/Portable/SemanticModelWorkspaceService/SemanticModelWorkspaceServiceFactory.cs

@@ -126,6 +126,11 @@ public async Task<SemanticModel> GetSemanticModelForNodeAsync(Document document,
                     if (oldMember == null)
                     {
                         // oops, something went wrong. we can't find old member. 
+                        //
+                        // due to how we do versioning (filestamp based versioning), there is always a possiblity that 
+                        // sources get changed without proper version changes in some rare situations,
+                        // so in those rare cases which we can't control until we move to content based versioning,
+                        // just bail out and use full semantic model
                         return await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
                     }