Abstract the secondary evac into a separate function

This is generic enough (no longer depending on the replace type) that
it will be used for other replace modes too. So all we need to do is
to abstract it to a separate function (that can be called from
multiple branches) and to make the imove op_fn computation more
generic, instead of hard-coding it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>