Simplify a condition in is_KXK()

No functional change.
Small renaming in material weights

Also dropped some temporary variable: compiler
is more than able to push on stack temp values
by itself (verified).

No functional change.
Move game_phase() to Position

It seems a more natural to place this
function there.

No functional change.
Better value clipping in game_phase()

No functional change.
Ensure ttValue != VALUE_NONE in singular extension search

The assert:

  assert(ttValue != VALUE_NONE);

Could fire for multiple reasons (although is very rare),
for instance after an IID we can have ttMove != MOVE_NONE
while ttValue is still set at VALUE_NONE.

But not only this, actually SMP is a source of corrupted
ttValue and anyhow we can detect the condition:

 ttMove != MOVE_NONE && ttValue == VALUE_NONE

even north of IID.

Reported by Ronald de Man.

It is so rare that bench didn't change.

bench: 7710548
Some reformatting in pawns.cpp

No functional change.