These three related patches were previously part of the AMX series,
which has been stuck for about 3 years. For AVX10 some of this is
going to be needed 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. Hence 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 is
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