Simplify Null Move Search Reduction

[BM123499]

This patch simplifies reduction on null move search, so it's more read-friendly.

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Previous Tests:
STC:
LLR: 2.97 (-2.94,2.94) <-2.50,0.50>
Total: 15632 W: 1319 L: 1204 D: 13109
Ptnml(0-2): 33, 956, 5733, 1051, 43
https://tests.stockfishchess.org/tests/view/60bd03c7457376eb8bcaa600

LTC:
LLR: 3.37 (-2.94,2.94) <-2.50,0.50>
Total: 86296 W: 2814 L: 2779 D: 80703
Ptnml(0-2): 33, 2500, 38039, 2551, 25
https://tests.stockfishchess.org/tests/view/60bd1ff0457376eb8bcaa653

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Current Tests:
STC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 23936 W: 1895 L: 1793 D: 20248
Ptnml(0-2): 40, 1470, 8869, 1526, 63
https://tests.stockfishchess.org/tests/view/611f9b7d4977aa1525c9cb6b

LTC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 62568 W: 1750 L: 1713 D: 59105
Ptnml(0-2): 19, 1560, 28085, 1605, 15
https://tests.stockfishchess.org/tests/view/611fa4814977aa1525c9cb71

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Functional on high depth.

[12345ieee]

While it does look better, the next tuning run will likely put the / largeNumber back to get finer tuning.

[BM123499]

I see... but, if the tuning passes, I will, naturally, retest this patch. This patch seems nicer to read.

[MichaelB7]

I agree it looks easier to read, but I would be interested in seeing how this does, staying a litte truer to the original code.
Depth R = std::min(int(eval - beta) / 205, 3) + (68 + 5 * depth)/16;

[ddobbelaere]

I disagree that it's nicer to read. I like the larger (and more precise/more tuned) numbers more.

A user cares about speed/elo, not subjective visual aesthetics of constants in the source code :)

[BM123499]

@ddobbelaere, what do you mean by "more precise/more tuned"? those variable are tuned in LTC, the equation deviates when depth = 18, 21, 24, 27, 30, .... how could we be sure which is more tuned? we would need a VVVLTC to search deeper, but the current was just tuned until LTC.
And aesthetics on the code are for programmers, if we didn't care about it, the code wouldn't have frequently clean up to be easier to read, or to remake the format, or to explain parts of the code. :)

[ddobbelaere]

@BM123499 Correct me if I'm wrong, but the current values (in master) are tuned at LTC, right? Then the version of this PR is just an approximation of the tuned formula, right, solely for aesthetic purposes?

Or am I missing something?

[BM123499]

This patch was meant for aesthetic purpose, but it doesn't mean it's inferior to master's formula.

[ddobbelaere]

Hmm yeah it can also be considered a simplification in terms of number of assembler instructions and hence, probably slightly faster. I didn't think about that in the first place...

See https://godbolt.org/z/zP3arvssT

[vondele]

somehow this patch fell through the cracks. Can you retest on current master, or otherwise close?

[BM123499]

Updated