1.0.28.20: further array typechecking optimization

 * Eliminate extra lowtag checking from the array element type check:
   since we know we are dealing with an array, we can use the same
   fast widetag extraction code that array type dispatching used --
   factoring it out into %OTHER-POINTER-WIDETAG.

 * If we know after checking the dimensions that the array must have
   a header, and we know that the array is simple, we can deduce that
   there is exactly one level of indirection.

 * Similarly, if we know that the array has a header, we can
   immediately pull out the data vector to check if it too has a
   header instead of doing an extra test.