Change root visitor to use Object**.

Simplifies code and improves the performance of root visiting since
we usually don't need to check to see if the object moved.

Change-Id: Iba998f5a15ae1fa1b53ca5226dd2168a411196cf
diff --git a/runtime/stack.h b/runtime/stack.h
index 2d56a74..7e9889e 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -565,6 +565,7 @@
   void SetVReg(mirror::ArtMethod* m, uint16_t vreg, uint32_t new_value, VRegKind kind)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
+  uintptr_t* GetGPRAddress(uint32_t reg) const;
   uintptr_t GetGPR(uint32_t reg) const;
   void SetGPR(uint32_t reg, uintptr_t value);