benchmark

[NFC] `complexity_n` is not of `IterationCount` type (#1709)

There is no bug here, but it gave me a scare the other day.
It is not incorrect to use `IterationCount` here,
since it's just an `int64_t` either way,
but it's wildly confusing. Let's not do that.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>