[FIX] expression: avoid useless query when searching on x2many sub-field

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!