Simplify the processing a leaf mask to find big-enough ranges of set

Simplify the processing a leaf mask to find big-enough ranges of set
bits, by storing and modifying the complement of the original leaf
mask, and by avoiding some unnecessary intermediate variables in
computing the shift amounts. The logic is similar to what has recently
been committed to sys/sys/bitstring.h.

Compute better hint updates for the case when the cursor starts in
mid-leaf, and eliminates some otherwise viable solutions. Assume the
worst case, that all the eliminated offsets could have been solutions,
and you can still compute a better hint than we use now.

Eliminate some unnecessary conditional control flow.

Approved by: alc
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22666