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/interpreter/mterp/mterp.cc b/runtime/interpreter/mterp/mterp.cc
index 8c372d4..6a8f864 100644
--- a/runtime/interpreter/mterp/mterp.cc
+++ b/runtime/interpreter/mterp/mterp.cc
@@ -773,7 +773,7 @@
     }
     ObjPtr<mirror::Object> obj = kIsStatic
         ? reinterpret_cast<ArtField*>(tls_value)->GetDeclaringClass()
-        : MakeObjPtr(shadow_frame->GetVRegReference(inst->VRegB_22c(inst_data)));
+        : ObjPtr<mirror::Object>(shadow_frame->GetVRegReference(inst->VRegB_22c(inst_data)));
     if (LIKELY(obj != nullptr)) {
       MterpFieldAccess<PrimType, kAccessType>(
           inst, inst_data, shadow_frame, obj, MemberOffset(offset), /* is_volatile= */ false);