Precompute clusterMask to speed up TranspositionTable::first_entry() whi...

...ch is on he hot path.

Timings:
gcc 4.9.1
AMD Phenom II
x86-64-modern

Master
4274
4259
4274
4275
4274
Average w/o min & max: 4274

Patch
4243
4243
4227
4228
4228
Average w/o min & max: 4233

1% speedup