x86-64/Intel: issue diagnostic for out of range displacement

x86-64/Intel: issue diagnostic for out of range displacement

... rather than silently dropping it altogether.
i386_finalize_displacement() expects baseindex to already be set, so
the respective statement needs to be moved up. This then also allows a
subsequent conditional to be simplified.

For this to not regress on 32-bit addressing, break out address size
guessing from i386_index_check(), invoking the new function earlier so
that i386_finalize_displacement() has i.prefix[ADDR_PREFIX] available.
i386_addressing_mode () in turn needs i.base_reg / i.index_reg set
earlier.