Fix domain removal when the root instruction is an empty domain

If a domain become empty because the various optimizations removed all
instruction from it then we have to re-add some instruction to make sure
the user supplied sharding is still respected.

This is especially important for the root instruction as the user will
expect the data to be available on the device they requested it. Before
this CL we failed to insert the tuple->gte sequence into the empty
domain due to a bug where we only considered cases where we have an exit
domain what is not the case for the root instruction.

PiperOrigin-RevId: 203744534