Turn FastNewContextStub into a HydrogenCodeStub.

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 fb382d13c1056c624d25d46f750dc3be08b769b6..64df73fe4f89122ed907c5cb31a2d72d1e8abcf1 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -182,20 +182,22 @@ void FullCodeGenerator::Generate() {
   if (heap_slots > 0) {
     Comment cmnt(masm_, "[ Allocate context");
     // Argument to NewContext is the function, which is still in rdi.
-    __ push(rdi);
     if (FLAG_harmony_scoping && info->scope()->is_global_scope()) {
+      __ push(rdi);
       __ Push(info->scope()->GetScopeInfo());
       __ CallRuntime(Runtime::kNewGlobalContext, 2);
     } else if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(heap_slots);
       __ CallStub(&stub);
     } else {
+      __ push(rdi);
       __ CallRuntime(Runtime::kNewFunctionContext, 1);
     }
     function_in_register = false;
-    // Context is returned in both rax and rsi.  It replaces the context
-    // passed to us.  It's saved in the stack and kept live in rsi.
-    __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rsi);
+    // Context is returned in rax.  It replaces the context passed to us.
+    // It's saved in the stack and kept live in rsi.
+    __ movp(rsi, rax);
+    __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rax);
 
     // Copy any necessary parameters into the context.
     int num_parameters = info->scope()->num_parameters();