s390: introduce DUMY opcode for special use.

src/s390/assembler-s390.h

Index: src/s390/assembler-s390.h
diff --git a/src/s390/assembler-s390.h b/src/s390/assembler-s390.h
index 60b45fbd2fa212d4e1082e7fe1e60c7e1f561892..10cbaa8f0d6d7b832746f4b4c45e7bc8e2ecfc3a 100644
--- a/src/s390/assembler-s390.h
+++ b/src/s390/assembler-s390.h
@@ -1224,6 +1224,8 @@ class Assembler : public AssemblerBase {
 
   void nop(int type = 0);  // 0 is the default non-marking type.
 
+  void dumy(int r1, int x2, int b2, int d2);
+
   // Check the code size generated from label to here.
   int SizeOfCodeGeneratedSince(Label* label) {
     return pc_offset() - label->pos();