Refactor the two uses of SHIFT{16,32} not followed by DO_BO_UNPACK().

Refactor the two uses of SHIFT{16,32} not followed by DO_BO_UNPACK().

There is one place where SHIFT16() is not immediately followed by
DO_BO_UNPACK() ('n!' and 'v!'), and one place where SHIFT32() is not
immediately followed by DO_BO_UNPACK() ('N!' and 'V!'). Every other use of
either macro is followed by DO_BO_UNPACK(). For the two locations that don't
have DO_BO_UNPACK(), the modifiers '<' and '>' are illegal, hence
DO_BO_UNPACK() will always be a no-op. Hence add DO_BO_UNPACK() to both
(which permits refactoring to simplify things), and add assert()s that the
assumptions are true.