Port r7868 (constant masking) to x64.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index a55788d10668a6ee9a97d57d3789f4ed5e5867c8..050c0d657ef91fcfc476c870f3d3ce0b17241569 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -504,12 +504,20 @@ void FullCodeGenerator::EffectContext::Plug(Handle<Object> lit) const {
 
 void FullCodeGenerator::AccumulatorValueContext::Plug(
     Handle<Object> lit) const {
-  __ Move(result_register(), lit);
+  if (lit->IsSmi()) {
+    __ SafeMove(result_register(), Smi::cast(*lit));
+  } else {
+    __ Move(result_register(), lit);
+  }
 }
 
 
 void FullCodeGenerator::StackValueContext::Plug(Handle<Object> lit) const {
-  __ Push(lit);
+  if (lit->IsSmi()) {
+    __ SafePush(Smi::cast(*lit));
+  } else {
+    __ Push(lit);
+  }
 }