[ast] Clean up a few unnecessary bits in AST

src/crankshaft/hydrogen.cc

Index: src/crankshaft/hydrogen.cc
diff --git a/src/crankshaft/hydrogen.cc b/src/crankshaft/hydrogen.cc
index 8b795ea724193741f564cfe1329f6475a6a957bf..ded0e99d9112f986e4a2d5718eb02742b990951f 100644
--- a/src/crankshaft/hydrogen.cc
+++ b/src/crankshaft/hydrogen.cc
@@ -7085,7 +7085,7 @@ void HOptimizedGraphBuilder::VisitAssignment(Assignment* expr) {
       }
     }
 
-    if (proxy->IsArguments()) return Bailout(kAssignmentToArguments);
+    if (var->is_arguments()) return Bailout(kAssignmentToArguments);
 
     // Handle the assignment.
     switch (var->location()) {