Avoid read barriers for ArtMethod::GetDexFile

Shows up in pmd benchmark from Class::FindDeclaredDirectMethod and
Class::FindDeclaredVirtualMethod. There are still calls to
IsProxyMethod that could probably be eliminated.

ReadBarrier::Mark goes from 12.39% to 3.45% according to perf.

Test: test-art-host

Change-Id: I6a4f2fa2d68bf5f393f83b9b70e8d6fcc9dbdaa2