ART: Refactor GraphVisualizer attribute printing

This patch unifies the way GraphVisualizer prints instruction
attributes in preparation of changes to the Checker syntax.

Change-Id: I44e91e36c660985ddfe039a9f410fedc48b496ec
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index b1cb880..0a0902b 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -691,11 +691,11 @@
 }
 
 void CodeGeneratorARM64::DumpCoreRegister(std::ostream& stream, int reg) const {
-  stream << Arm64ManagedRegister::FromXRegister(XRegister(reg));
+  stream << XRegister(reg);
 }
 
 void CodeGeneratorARM64::DumpFloatingPointRegister(std::ostream& stream, int reg) const {
-  stream << Arm64ManagedRegister::FromDRegister(DRegister(reg));
+  stream << DRegister(reg);
 }
 
 void CodeGeneratorARM64::MoveConstant(CPURegister destination, HConstant* constant) {