stack-allocatable fill-initialized specialized arrays, take 2

stack-allocatable fill-initialized specialized arrays, take 2

  Really fix lp#902537.

  Turns out multiple references to the stack allocated vector complicate
  things /just/ sufficiently the that DX machinery can't keep up. (I because
  the code that allocates and initializes the vector isn't substituted at the
  use-site due to multiple references.)

  While it would be nice to make it smart enough to deal with
  non-let-converted WITH-ARRAY-DATA, that looks a bit tricky... so instead
  simplify things in the FILL transform when the vector is a simple-array.