Remove one test in the move loop

[snicolet]

Simplification passed STC test:
https://tests.stockfishchess.org/tests/view/6519fc91cff46e538ee014f6
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 191264 W: 48550 L: 48501 D: 94213
Ptnml(0-2): 576, 21529, 51392, 21540, 595

closes https://github.com/snicolet/Stockfish/issues/4815

Non functional change

[Torom]

ARCH=x86-64-bmi2 clang 16.0.6

Result of  50 runs
==================
base (...es/stockfish) =     911014  +/- 2646
test (...ish.legality) =     907976  +/- 3081
diff                   =      -3038  +/- 4366

speedup        = -0.0033
P(speedup > 0) =  0.0866

CPU: 4 x Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
Hyperthreading: on

ARCH=x86-64-bmi2 gcc 13.2.1

Result of  50 runs
==================
base (...tockfish_gcc) =     902561  +/- 2624
test (...legality_gcc) =     900882  +/- 3032
diff                   =      -1679  +/- 4673

speedup        = -0.0019
P(speedup > 0) =  0.2410

CPU: 4 x Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
Hyperthreading: on

[ddobbelaere]

If anything, I think this patch should be a slowdown, as @Torom also experimentally shows. Note that rootNode is constexpr, hence the compiler knows its value based on the template argument and will never infer extra logic.

But on root nodes, the "legal position check" on master is skipped, as we know the position is legal there anyway (see comments directly above).

Hence, all that this PR does is add an extra unnecessary legality check for root nodes.

I have to acknowledge @peregrineshahin for mentioning this first on Discord.