[Attributor] Fix -Wunused-variable in -DLLVM_ENABLE_ASSERTIONS=off builds after r369331

llvm-svn: 369334

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -2552,6 +2552,7 @@ ChangeStatus Attributor::run() {
   NumAttributesManifested += NumManifested;
   NumAttributesValidFixpoint += NumAtFixpoint;
 
+  (void)NumFinalAAs;
   assert(
       NumFinalAAs == AllAbstractAttributes.size() &&
       "Expected the final number of abstract attributes to remain unchanged!");