Add more options for `BM_iteration` in order to see better picture for choosing trade off for iteration optimizations.

```
BM_Iteration/1/1      1.83ns ± 0%
BM_Iteration/2/2      2.63ns ±11%
BM_Iteration/4/4      5.76ns ±26%
BM_Iteration/7/7      3.79ns ± 0%
BM_Iteration/10/10    8.49ns ±23%
BM_Iteration/15/15    18.2ns ±30%
BM_Iteration/16/16    21.2ns ±29%
BM_Iteration/54/54    37.2ns ±21%
BM_Iteration/100/100  74.7ns ±13%
BM_Iteration/400/400   330ns ± 8%
BM_Iteration/0/0      0.46ns ± 2%
BM_Iteration/10/0     1.26ns ± 1%
BM_Iteration/100/0    13.4ns ± 0%
BM_Iteration/1000/0    417ns ± 0%
BM_Iteration/10000/0  3.30µs ± 0%
BM_Iteration/100/1    16.0ns ±12%
BM_Iteration/1000/10   453ns ± 5%
```

PiperOrigin-RevId: 474282700
Change-Id: I4b3fcb80292147aa4a8f542ae5c7fc1e8bd5f05b

absl/container/internal/raw_hash_set_benchmark.cc

@@ -12,7 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <array>
+#include <cmath>
 #include <numeric>
+#include <utility>
 #include <random>
 #include <vector>
 
@@ -235,10 +238,22 @@ void BM_Iteration(benchmark::State& state) {
 }
 
 BENCHMARK(BM_Iteration)
+    ->ArgPair(1, 1)
+    ->ArgPair(2, 2)
+    ->ArgPair(4, 4)
+    ->ArgPair(7, 7)
     ->ArgPair(10, 10)
-    ->ArgPair(20, 20)
+    ->ArgPair(15, 15)
+    ->ArgPair(16, 16)
+    ->ArgPair(54, 54)
     ->ArgPair(100, 100)
     ->ArgPair(400, 400)
+    // empty
+    ->ArgPair(0, 0)
+    ->ArgPair(10, 0)
+    ->ArgPair(100, 0)
+    ->ArgPair(1000, 0)
+    ->ArgPair(10000, 0)
     // sparse
     ->ArgPair(100, 1)
     ->ArgPair(1000, 10);