X64 Crankshaft: Implement LValueOf and (dummy) LArgumentsObject.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
===================================================================
--- src/arm/lithium-arm.cc	(revision 6847)
+++ src/arm/lithium-arm.cc	(working copy)
@@ -1918,8 +1918,10 @@
 
 
 LInstruction* LChunkBuilder::DoArgumentsObject(HArgumentsObject* instr) {
-  // There are no real uses of the arguments object (we bail out in all other
-  // cases).
+  // There are no real uses of the arguments object.
+  // arguments.length and element access are supported directly on
+  // stack arguments, and any real arguments object use causes a bailout.
+  // So this value is never used.
   return NULL;
 }