vpack/vunpack from PackedSIMD VSX is needed in SVP64.
it can be implemented using REMAP in an advanced form
and as a special SVP64 MV-RM Mode providing an
extra (2nd) SUBVL.
from the original discussion (going back a long way) this part of the
spec has morphed into some "RM EXTRA" fields for Pack and Unpack.
there is also a REMAP mode based on Matrix.