1.0.28.55: transform FILL to a UB*-BASH-FILL when possible

1.0.28.55: transform FILL to a UB*-BASH-FILL when possible

 The performance boost for all cases which previously used
 VECTOR-FILL* is quite noticeable.

 Also delay the FILL transform if the vector element type is
 not yet known.

 ...also one leftover #+sb-xc-host from the previous commit.