[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 892bff674f6426ba61e88120e7b4a0a563425908..f8276d6f1f2fc237797b441e080f01813f04f829 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -1568,6 +1568,11 @@ inline Operand ContextOperand(Register context, int index) {
 }
 
 
+inline Operand ContextOperand(Register context, Register index) {
+  return Operand(context, index, times_pointer_size, Context::SlotOffset(0));
+}
+
+
 inline Operand GlobalObjectOperand() {
   return ContextOperand(rsi, Context::GLOBAL_OBJECT_INDEX);
 }