Make guard_cid and nullable_cid half words.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index e47d0cfb0768a3456e54ee4043e4002b032ebfaf..970df88c887b50238906c2bd576c552d2c3a6606 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -397,6 +397,7 @@ class Assembler : public ValueObject {
   void movsxw(Register dst, const Address& src);
   void movw(Register dst, const Address& src);
   void movw(const Address& dst, Register src);
+  void movw(const Address& dst, const Immediate& imm);
 
   void movq(Register dst, const Immediate& imm);
   void movq(Register dst, Register src);
@@ -512,6 +513,9 @@ class Assembler : public ValueObject {
 
   void cmpb(const Address& address, const Immediate& imm);
 
+  void cmpw(Register reg, const Address& address);
+  void cmpw(const Address& address, const Immediate& imm);
+
   void cmpl(Register reg, const Immediate& imm);
   void cmpl(Register reg0, Register reg1);
   void cmpl(Register reg, const Address& address);