Searching on a domain like `[('m2m.sub', operator, value)]` currently does
something like:
right_ids = comodel.search([('sub', operator, value)]).ids
table_ids = model.search([('m2m', 'in', right_ids)]).ids
and reduces the domain triple to `('id', 'in', table_ids)`.
The domain triple can actually be reduced to `('m2m', 'in', right_ids)`. With
this reduction, the search on the field `m2m` will be done as part of the main
query. And this will also enable the optimization of the former fix!
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