Save static evaluation also for failed low nodes

Save static evaluation also for failed low nodes

When a node fails low and bestValue is still equal to
the original static node evaluation, then save this
in TT along with usual info.

This will allow us to avoid a future costly evaluation() call.

This patch extends to failed low nodes what we already do
for failed high ones.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>