Turn FastNewContextStub into a HydrogenCodeStub.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 30f6e05257695d59825d9a2e5f578ec9eb4b5f7e..e99c9e35cd533e65fcfcb57d9f35bf0ecea23ddb 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -218,17 +218,18 @@ bool LCodeGen::GeneratePrologue() {
   if (heap_slots > 0) {
     Comment(";;; Allocate local context");
     // Argument to NewContext is the function, which is still in rdi.
-    __ push(rdi);
     if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(heap_slots);
       __ CallStub(&stub);
     } else {
+      __ push(rdi);
       __ CallRuntime(Runtime::kNewFunctionContext, 1);
     }
     RecordSafepoint(Safepoint::kNoLazyDeopt);
-    // 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 = scope()->num_parameters();