[lld/mac] Tweak estimateStubsInRangeVA a bit
- Move a few variables closer to their uses, remove some completely
(no behavior change)
- Add some comments
- Make maxPotentialThunks include calls to stubs. It's possible that
an earlier call to a stub late in the stub table will need a thunk,
and that inserted thunk could push a stub earlier in the stub table
out of range. This is unlikely to happen, but usually there are
way fewer stub calls than non-stub calls, so if we're doing a
conservative approximation here we might as well do it correctly.
(For chromium's unit_tests target, 134421/242639 stub calls are
direct calls without this change, compared to 134408/242639 with
this change)
No real, meaningful behavior difference.
Differential Revision: https://reviews.llvm.org/D108924