This converts the slice_mask bit operation helpers to be the usual
3-operand kind, which allows 2 inputs to set a different output
without an extra copy, which is used in the next patch.

Adds slice_copy_mask, which will be used in the next patch.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>