[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
diff --git a/src/ia32/macro-assembler-ia32.h b/src/ia32/macro-assembler-ia32.h
index d4d4fd6b90b44966c9c60be8519b7ddeff12f61e..15dbaed69d650b4d3a079df68e8f0999d51a8dc7 100644
--- a/src/ia32/macro-assembler-ia32.h
+++ b/src/ia32/macro-assembler-ia32.h
@@ -1068,6 +1068,11 @@ 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(esi, Context::GLOBAL_OBJECT_INDEX);
 }