Use the ability to invert phase with ms_mask instead of changing
the codebook when possible, to avoid having to switch codebooks
if some bands are INTENSTY_BT and others are INTENSITY_BT2, since
usually a set ms_mask uses less bits that a codebook change. While
it may not always be a win (ie: if it causes an ms_mask bitmap
to be sent when it wouldn't have been otherwise), it's unlikely
since the ms_mask bitmap will almost always be there already for
M/S itself.