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_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index e633970..63d6846 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -396,11 +396,11 @@
 }
 
 void CodeGeneratorX86_64::DumpCoreRegister(std::ostream& stream, int reg) const {
-  stream << X86_64ManagedRegister::FromCpuRegister(Register(reg));
+  stream << Register(reg);
 }
 
 void CodeGeneratorX86_64::DumpFloatingPointRegister(std::ostream& stream, int reg) const {
-  stream << X86_64ManagedRegister::FromXmmRegister(FloatRegister(reg));
+  stream << FloatRegister(reg);
 }
 
 size_t CodeGeneratorX86_64::SaveCoreRegister(size_t stack_index, uint32_t reg_id) {