Ancestral Spirit vs Silence, Mass Dispel, etc

Is Ancestral Spirit considered a 'Deathrattle' effect to the original card, so no matter what happens prior to it's death it returns in it's original form?

Or will Silence and Mass Dispel remove it, thus making it a lost buff to the card?

Or if it can't be removed once cast, will transformations persist through death, like Polymorph and Hex? So when it does return, it is a sheep or frog instead of Deathwing?

The topic has been brought up before, but would love to get some official clarification if possible.

Thanks!