Small simplification to passed pawns

Tested in no-regression mode.

Passed both STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 28521 W: 5066 L: 4958 D: 18497

And LTC
LLR: 3.04 (-2.94,2.94) [-3.00,1.00]
Total: 21939 W: 3261 L: 3138 D: 15540

bench: 8165681
Generalize shift_bb() to handle double pushes

And use it in evaluate_space.

No functional change.