Loader.hs: set instance auto-repair policy in mergeData

'getArPolicy' and 'setArPolicy' follow the precedence rules introduced in
b1eb71c7: within an object, the most restrictive tag wins; across object,
the nearest tag wins.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>