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

llvm-svn: 369334
diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 0e86efd..4cdb385 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -2552,6 +2552,7 @@
   NumAttributesManifested += NumManifested;
   NumAttributesValidFixpoint += NumAtFixpoint;
 
+  (void)NumFinalAAs;
   assert(
       NumFinalAAs == AllAbstractAttributes.size() &&
       "Expected the final number of abstract attributes to remain unchanged!");