Reduce register pressure on ARM, ARM64, and MIPS.

runtime/vm/assembler_arm.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_ARM_H_
 #define VM_ASSEMBLER_ARM_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_arm.h directly; use assembler.h instead.
 #endif
@@ skipping 704 matching lines @@
   void Vdivqs(QRegister qd, QRegister qn, QRegister qm);
 
   void SmiTag(Register reg, Condition cond = AL) {
     Lsl(reg, reg, kSmiTagSize, cond);
   }
 
   void SmiUntag(Register reg, Condition cond = AL) {
     Asr(reg, reg, kSmiTagSize, cond);
   }
 
+  void SmiUntag(Register dst, Register src, Condition cond = AL) {
+    Asr(dst, src, kSmiTagSize, cond);
+  }
+
   // Function frame setup and tear down.
   void EnterFrame(RegList regs, intptr_t frame_space);
   void LeaveFrame(RegList regs);
   void Ret();
   void ReserveAlignedFrameSpace(intptr_t frame_space);
 
   // Create a frame for calling into runtime that preserves all volatile
   // registers.  Frame's SP is guaranteed to be correctly aligned and
   // frame_space bytes are reserved under it.
   void EnterCallRuntimeFrame(intptr_t frame_space);
@@ skipping 186 matching lines @@
                                   Register value,
                                   Label* no_update);
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(Assembler);
 };
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_ARM_H_