stack-allocatable fill-initialized specialized arrays

stack-allocatable fill-initialized specialized arrays

 I *think* we had this working earlier already, but it's been broken at least
 for a while now since there were no tests for it.

 Add a DEFKNOWN to the array byte bashers, providing the RESULT-ARG -- and
 make them return the sequence.

 Replace the unused and bitrotted UNSAFE IR1 attribute with its inverse:
 DX-SAFE, and use that togather with RESULT-ARG to allow multiple refs to
 potentially DX leafs. Still accept UNSAFE in DEFKNOWNs occurring in
 user-code, but ignore it and give a style-warning.

 For now, add DX-SAFE to LENGTH and VECTOR-LENGTH, which is enough for our
 purposes.

 Fixes lp#902351.