Start using the overwrite mode from the full codegens to generate...

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
===================================================================
--- src/x64/codegen-x64.cc	(revision 5338)
+++ src/x64/codegen-x64.cc	(working copy)
@@ -5009,9 +5009,7 @@
     Load(node->value());
 
     // Perform the binary operation.
-    bool overwrite_value =
-        (node->value()->AsBinaryOperation() != NULL &&
-         node->value()->AsBinaryOperation()->ResultOverwriteAllowed());
+    bool overwrite_value = node->value()->ResultOverwriteAllowed();
     // Construct the implicit binary operation.
     BinaryOperation expr(node);
     GenericBinaryOperation(&expr,
@@ -5100,9 +5098,7 @@
     frame()->Push(&value);
     Load(node->value());
 
-    bool overwrite_value =
-        (node->value()->AsBinaryOperation() != NULL &&
-         node->value()->AsBinaryOperation()->ResultOverwriteAllowed());
+    bool overwrite_value = node->value()->ResultOverwriteAllowed();
     // Construct the implicit binary operation.
     BinaryOperation expr(node);
     GenericBinaryOperation(&expr,
@@ -5202,9 +5198,7 @@
     Load(node->value());
 
     // Perform the binary operation.
-    bool overwrite_value =
-        (node->value()->AsBinaryOperation() != NULL &&
-         node->value()->AsBinaryOperation()->ResultOverwriteAllowed());
+    bool overwrite_value = node->value()->ResultOverwriteAllowed();
     BinaryOperation expr(node);
     GenericBinaryOperation(&expr,
                            overwrite_value ? OVERWRITE_RIGHT : NO_OVERWRITE);
@@ -7357,9 +7351,7 @@
     }
 
   } else {
-    bool can_overwrite =
-      (node->expression()->AsBinaryOperation() != NULL &&
-       node->expression()->AsBinaryOperation()->ResultOverwriteAllowed());
+    bool can_overwrite = node->expression()->ResultOverwriteAllowed();
     UnaryOverwriteMode overwrite =
         can_overwrite ? UNARY_OVERWRITE : UNARY_NO_OVERWRITE;
     bool no_negative_zero = node->expression()->no_negative_zero();
@@ -7777,11 +7769,9 @@
     // NOTE: The code below assumes that the slow cases (calls to runtime)
     // never return a constant/immutable object.
     OverwriteMode overwrite_mode = NO_OVERWRITE;
-    if (node->left()->AsBinaryOperation() != NULL &&
-        node->left()->AsBinaryOperation()->ResultOverwriteAllowed()) {
+    if (node->left()->ResultOverwriteAllowed()) {
       overwrite_mode = OVERWRITE_LEFT;
-    } else if (node->right()->AsBinaryOperation() != NULL &&
-               node->right()->AsBinaryOperation()->ResultOverwriteAllowed()) {
+    } else if (node->right()->ResultOverwriteAllowed()) {
       overwrite_mode = OVERWRITE_RIGHT;
     }