Simplify evaluation for blocked passers.

This is a functional simplification of the evaluation code for blocked passers.
I've also changed a few variable names for clarity.

STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 141984 W: 27450 L: 27466 D: 87068
Ptnml(0-2): 2414, 16511, 33175, 16461, 2431
https://tests.stockfishchess.org/tests/view/5ec4001b05aa4bc72d9759e7

LTC
LLR: 2.93 (-2.94,2.94) {-1.50,0.50}
Total: 30536 W: 3966 L: 3885 D: 22685
Ptnml(0-2): 216, 2841, 9073, 2922, 216
https://tests.stockfishchess.org/tests/view/5ec4bd0d377121ac09e101b7

Closes https://github.com/official-stockfish/Stockfish/pull/2690

Bench: 4704681