Fix issue 13748 -- benchmark druntime/benchmark/aabench/string.d fails

When max node threshold forced a rehash, the `firstUsedBucket` cache was not updated before doing the rehash. Since `firstUsedBucket` was used to avoid searching through known null elements when copying nodes to the new bucket array, if the newly added element was *before* `firstUsedBucket`, then it was skipped and lost.