Make guard_cid and nullable_cid half words.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 804dcf21a18652018d240d6ac95f420bedfe4f42..6984fc3ab35c7a1507f1543830bc8be8cb632b77 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -350,6 +350,16 @@ void Assembler::movw(const Address& dst, Register src) {
 }
 
 
+void Assembler::movw(const Address& dst, const Immediate& imm) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitOperandSizeOverride();
+  EmitUint8(0xC7);
+  EmitOperand(0, dst);
+  EmitUint8(imm.value() & 0xFF);
+  EmitUint8((imm.value() >> 8) & 0xFF);
+}
+
+
 void Assembler::movq(Register dst, const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   if (imm.is_int32()) {
@@ -1419,6 +1429,24 @@ void Assembler::cmpb(const Address& address, const Immediate& imm) {
 }
 
 
+void Assembler::cmpw(Register reg, const Address& address) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitOperandSizeOverride();
+  EmitUint8(0x3B);
+  EmitOperand(reg, address);
+}
+
+
+void Assembler::cmpw(const Address& address, const Immediate& imm) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitOperandSizeOverride();
+  EmitUint8(0x81);
+  EmitOperand(7, address);
+  EmitUint8(imm.value() & 0xFF);
+  EmitUint8((imm.value() >> 8) & 0xFF);
+}
+
+
 void Assembler::cmpl(Register reg, const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitRegisterREX(reg, REX_NONE);