Add moves from failed high nodes in PV

Add moves from failed high nodes in PV

Considering only VALUE_TYPE_EXACT nodes is too
restrictive and has a number of side-effects, most
notably the truncation of PV line after a fail high
at root.

Note that in this way we are no more guaranteed that
PV line is built up with PV nodes only, because
it could happen that a side search overwrites with a
cut-off move a PV node and this cut-off move ends up
in PV.

Change should be almost not measurable, perhaps with
ponder on we could have some beneficial effect.

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