VM: Improve receiver class check in polymorphic inlining.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
===================================================================
--- runtime/vm/assembler_x64.cc	(revision 38058)
+++ runtime/vm/assembler_x64.cc	(working copy)
@@ -2124,7 +2124,16 @@
   EmitUint8(0xD0 | (reg & 7));
 }
 
+void Assembler::btq(Register base, Register offset) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  Operand operand(base);
+  EmitOperandREX(offset, operand, REX_W);
+  EmitUint8(0x0F);
+  EmitUint8(0xA3);
+  EmitOperand(offset & 7, operand);
+}
 
+
 void Assembler::enter(const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0xC8);