Destination flipping for add!!(x, y)

[oxinabox]

Right now add!!(x, y) will only ever mutate x, never y.
because of the logically linear nature of AD, y being a differential (i.e. the output of pullback) means it is used exactly once .
Which means we are infact free to mutate it, also since this will be it's only use. (same also says why we are always ok to mutate x)
See google-research/dex-lang#401 (comment)
(and to a lesser extent https://twitter.com/oxinabox_frames/status/1387722380978622468)

So we could pick either x or y depending on which was a valid inplacable_destination.
And maybe also always chose Array if one of them is an Array (since that is the best destination if that has been allocated)

This would solve at least the basic cases of #345

[oxinabox]

Argument agaist this is that if y comes from identity_pullback(y)=y
and was provided by the user then the user might be using it for something else also.

Kind feel like that shouldn't be our problem though.