[llvm] [llvm-exegesis] Add middle half repetition mode (PR #77020)

[Clement Courbet via llvm-commits]

================
@@ -455,6 +466,26 @@ static void runBenchmarkConfigurations(
               Measurement.PerSnippetValue, NewMeasurement.PerSnippetValue);
         }
       }
+    } else if (RepetitionMode ==
----------------
legrosbuffle wrote:

This shared a lot of code with the `if (RepetitionMode == Benchmark::RepetitionModeE::AggregateMin) {` above.

This could probably be:

```
if (RepetitionMode == Benchmark::RepetitionModeE::AggregateMin) {
  Result = AggregateResults(AllResults, []() { ... });
} else {
  Result = AggregateResults(AllResults, []() { ... });
}
```

https://github.com/llvm/llvm-project/pull/77020