Implements the two argument shuffle SIMD instruction for ARM.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
===================================================================
--- runtime/vm/assembler_arm.h	(revision 25620)
+++ runtime/vm/assembler_arm.h	(working copy)
@@ -538,6 +538,10 @@
   // a list of 'length' registers starting with dn. The result is placed in dd.
   void vtbl(DRegister dd, DRegister dn, int length, DRegister dm);
 
+  // The words of qd and qm are interleaved with the low words of the result
+  // in qd and the high words in qm.
+  void vzipqw(QRegister qd, QRegister qm);
+
   // Branch instructions.
   void b(Label* label, Condition cond = AL);
   void bl(Label* label, Condition cond = AL);