Forbid the use of shifts in ShifterOperand in Thumb2

Change-Id: I4b3b4d90f11f2dd0c9a274b724051fa98982fb19
diff --git a/compiler/utils/arm/assembler_arm.h b/compiler/utils/arm/assembler_arm.h
index 8730f52..dd0dba2 100644
--- a/compiler/utils/arm/assembler_arm.h
+++ b/compiler/utils/arm/assembler_arm.h
@@ -100,6 +100,10 @@
     return rm_;
   }
 
+  Register GetSecondRegister() const {
+    return rs_;
+  }
+
   enum Type {
     kUnknown = -1,
     kRegister,