Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/mirror/object-inl.h b/runtime/mirror/object-inl.h
index b89da9d..c9e60bc 100644
--- a/runtime/mirror/object-inl.h
+++ b/runtime/mirror/object-inl.h
@@ -121,7 +121,7 @@
       OFFSET_OF_OBJECT_MEMBER(Object, x_rb_ptr_));
 #else
   LOG(FATAL) << "Unreachable";
-  return nullptr;
+  UNREACHABLE();
 #endif
 }
 
@@ -134,6 +134,7 @@
       OFFSET_OF_OBJECT_MEMBER(Object, x_rb_ptr_), rb_ptr);
 #else
   LOG(FATAL) << "Unreachable";
+  UNREACHABLE();
 #endif
 }
 
@@ -156,7 +157,7 @@
   return true;
 #else
   LOG(FATAL) << "Unreachable";
-  return false;
+  UNREACHABLE();
 #endif
 }
 
@@ -166,13 +167,12 @@
     DCHECK(obj->GetReadBarrierPointer() == nullptr)
         << "Bad Baker pointer: obj=" << reinterpret_cast<void*>(obj)
         << " ptr=" << reinterpret_cast<void*>(obj->GetReadBarrierPointer());
-  } else if (kUseBrooksReadBarrier) {
+  } else {
+    CHECK(kUseBrooksReadBarrier);
     Object* obj = const_cast<Object*>(this);
     DCHECK_EQ(obj, obj->GetReadBarrierPointer())
         << "Bad Brooks pointer: obj=" << reinterpret_cast<void*>(obj)
         << " ptr=" << reinterpret_cast<void*>(obj->GetReadBarrierPointer());
-  } else {
-    LOG(FATAL) << "Unreachable";
   }
 }