compress is intented to match vcompress from the ISA manual. Note that
deinterleave is a subset of this, and is already tested elsewhere.
decompress is the synthetic pattern defined in same - though we can often
do better than the mentioned iota/vrgather. Note that some of these
can also be expressed as interleave with at least one undef source,
and is already tested elsewhere.
repeat repeats each input element N times in the output. It can be
described as as a interleave operations, but we can sometimes do
better lowering wise.
c55a080c08