Fix readability of TTEntry occupancy check

[dubslow]

I say "fix" due #5263. That discussion was lengthy, and at least one comment appeared to not know what this code did. I wonder if it would have been submitted (or accepted) if that patch had read more like this one.

Also, setting aside the old PR, this is a clear readability gain anyways. Beginners can't understand it all, intermediates need to think a while to recall it, and even experts need a couple seconds per read to work out if it's an occupancy check or a genuine depth computation.

I think it just makes the code clearly easier to read and maintain.

Passed STC non-reg: https://tests.stockfishchess.org/tests/view/66695b6a602682471b064cfc
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 107520 W: 28138 L: 27998 D: 51384
Ptnml(0-2): 373, 12257, 28358, 12401, 371

no functional change

[github-actions]

clang-format 18 needs to be run on this PR.
If you do not have clang-format installed, the maintainer will run it when merging.
For the exact version please see https://packages.ubuntu.com/noble/clang-format-18.

(execution 9523455495 / attempt 1)