Rewrite search best value update

Rewrite search best value update

A simplification and also a small speed-up of
about 1% mainly due to reducing calls to
thisThread->cutoff_occurred().

Worst case split point recovering time after a
cut-off occurred is limited to 3 msec on my slow
PC, and usually is below 1 msec, so it seems safe
to remove the cutoff_occurred() check.

No functional change.