Opt compiler: Add disassembly to the '.cfg' output.

This is automatically added to the '.cfg' output when using the usual
`--dump-cfg` option.

Change-Id: I864bfc3a8299c042e72e451cc7730ad8271e4deb
diff --git a/compiler/optimizing/code_generator_arm64.h b/compiler/optimizing/code_generator_arm64.h
index f96810f..bbe3adc 100644
--- a/compiler/optimizing/code_generator_arm64.h
+++ b/compiler/optimizing/code_generator_arm64.h
@@ -283,6 +283,7 @@
   HGraphVisitor* GetLocationBuilder() OVERRIDE { return &location_builder_; }
   HGraphVisitor* GetInstructionVisitor() OVERRIDE { return &instruction_visitor_; }
   Arm64Assembler* GetAssembler() OVERRIDE { return &assembler_; }
+  const Arm64Assembler& GetAssembler() const OVERRIDE { return assembler_; }
   vixl::MacroAssembler* GetVIXLAssembler() { return GetAssembler()->vixl_masm_; }
 
   // Emit a write barrier.