Clean up explicit conversions to ObjPtr<>.

Add an ObjPtr<>::DownCast() overload that takes a plain
pointer and remove unnecessary calls to MakeObjPtr(),
usually preceding DownCast(). Move the MakeObjPtr() to
common_art_test.h .

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: I2a243b6d8f3b2e773396dfc53b659c5f7d9ea44a
diff --git a/runtime/mirror/array-alloc-inl.h b/runtime/mirror/array-alloc-inl.h
index 4250ff8..2ae4cab 100644
--- a/runtime/mirror/array-alloc-inl.h
+++ b/runtime/mirror/array-alloc-inl.h
@@ -142,16 +142,16 @@
   ObjPtr<Array> result;
   if (!kFillUsable) {
     SetLengthVisitor visitor(component_count);
-    result = ObjPtr<Array>::DownCast(MakeObjPtr(
-        heap->AllocObjectWithAllocator<kIsInstrumented, true>(self, array_class, size,
-                                                              allocator_type, visitor)));
+    result = ObjPtr<Array>::DownCast(
+        heap->AllocObjectWithAllocator<kIsInstrumented, true>(
+            self, array_class, size, allocator_type, visitor));
   } else {
     SetLengthToUsableSizeVisitor visitor(component_count,
                                          DataOffset(1U << component_size_shift).SizeValue(),
                                          component_size_shift);
-    result = ObjPtr<Array>::DownCast(MakeObjPtr(
-        heap->AllocObjectWithAllocator<kIsInstrumented, true>(self, array_class, size,
-                                                              allocator_type, visitor)));
+    result = ObjPtr<Array>::DownCast(
+        heap->AllocObjectWithAllocator<kIsInstrumented, true>(
+            self, array_class, size, allocator_type, visitor));
   }
   if (kIsDebugBuild && result != nullptr && Runtime::Current()->IsStarted()) {
     array_class = result->GetClass();  // In case the array class moved.