These three related patches were previously part of the AMX series.
For AVX10 some of this is going to be needed (or at least wanted) too,
though, hence I'm now refreshing these separately. Neither of the
three is strictly a functional prereq, but two of the three introduce
new functions which then are further altered while adding AVX10 (and
AMX) logic. Presumably none of these would be necessary anymore once
toolstack side work planned by Andrew gets carried out.

All taken together: Until it is clear that one or more of these are
strictly not needed / wanted (in this or another form), having these
ahead of the actual work seems preferable over accidentally losing
adjustments done for AVX10 / AMX (and down the road perhaps others).

1: adjust extended leaves out of range clearing
2: shrink max_{,sub}leaf fields according to actual leaf contents
3: move bounding of max_{,sub}leaf fields to library code

Jan