Small clean up in art::x86_64::ReadBarrierMarkSlowPathX86_64.

Clearly separate art::x86_64::CpuRegister (register object
used by the x86-64 assembler) from art::x86_64::Register
(register number) in
art::x86_64::ReadBarrierMarkSlowPathX86_64::EmitNativeCode.

Test: ART_USE_READ_BARRIER=true make test-art-host
Bug: 12687968
Change-Id: I4300e9b9c16b18119d4e399092aa1c9543518ab5
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 665d028..5cabc8f 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -457,7 +457,8 @@
 
   void EmitNativeCode(CodeGenerator* codegen) OVERRIDE {
     LocationSummary* locations = instruction_->GetLocations();
-    Register reg = obj_.AsRegister<Register>();
+    CpuRegister cpu_reg = obj_.AsRegister<CpuRegister>();
+    Register reg = cpu_reg.AsRegister();
     DCHECK(locations->CanCall());
     DCHECK(!locations->GetLiveRegisters()->ContainsCoreRegister(reg));
     DCHECK(instruction_->IsInstanceFieldGet() ||
@@ -476,7 +477,7 @@
     __ Bind(GetEntryLabel());
     if (unpoison_) {
       // Object* ref = ref_addr->AsMirrorPtr()
-      __ MaybeUnpoisonHeapReference(obj_.AsRegister<CpuRegister>());
+      __ MaybeUnpoisonHeapReference(cpu_reg);
     }
     // No need to save live registers; it's taken care of by the
     // entrypoint. Also, there is no need to update the stack mask,