If the attributes don't exist, just bail for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172854 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/Attributes.cpp b/lib/IR/Attributes.cpp
index bb98358..5de1827 100644
--- a/lib/IR/Attributes.cpp
+++ b/lib/IR/Attributes.cpp
@@ -253,7 +253,7 @@
       break;
     }
 
-  assert(AWI && "Cannot find index in attribute set!");
+  if (!AWI) return;
 
   uint64_t Mask = AWI->Attrs.Raw();