Reland r38116: Improve receiver class check in polymorphic inlining.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
===================================================================
--- runtime/vm/assembler_ia32.cc	(revision 38118)
+++ runtime/vm/assembler_ia32.cc	(working copy)
@@ -1774,6 +1774,14 @@
 }
 
 
+void Assembler::bt(Register base, Register offset) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x0F);
+  EmitUint8(0xA3);
+  EmitRegisterOperand(offset, base);
+}
+
+
 void Assembler::enter(const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0xC8);