Basically Pack/Unpack covers everything that VSX `vpkpx` and
other ops can do, and then some: Saturation included, for arithmetic ops.
-# No Scalar GPR Move
-
-Perhaps unsurprisingly the Scalar Power ISA does not have
-a Scalar GPR Move instruction: instead, there are a series
-of pseudo-op opportunities such as `addi RT,RA,0` or `ori RT,RA,0`
-and many more.
-
-Strictly speaking these may orthogonally be Vectorised and achieve
-the same effect as a Vector Move. However these instructions
-are marked as `RM-2P-1S1D` and have EXTRA3 Augmentation. In other
-words it is not possible to use them in Pack/Unpack Mode.
-There is however a trick: [[sv/mv.swizzle]] with a straight linear
-mapping (X to X, Y to Y...)
-By applying a straight linear swizzle map, the `RM-2P-1S1D-PU` mode
-of `sv.mv.swizzle`
-is available.
-
-It has however been decided to make one of the many pseudo-op aliases
-a Pack/Unpack variant: `sv.xori RT,RA,0`. This loses half the range
-of access of the Scalar regs (r0..r63) due to using an EXTRA2. It was
-felt to be better to do this to xori rather than ori or addi.
-
-Pack/Unpack has to be deployed across SVP64 sparingly (not so uniformly
-general) due to the fact that it takes up two RM.EXTRA bits, putting
-pressure on developers by restricting the register range as above.
-
# LD/ST with zero-immediate vs mapreduce mode
LD/ST operations with a zero immediate effectively means that on a