[llvm-exegesis] Let Counter returns up to 16 entries

    LBR contains (up to) 16 entries for last x branches and the X86LBRCounter (from D77422) should be able to return all those.
    Currently, it just returns the latest entry, which could lead to mis-leading measurements.
    This patch aslo changes the LatencyBenchmarkRunner to accommodate multi-value readings.

         https://reviews.llvm.org/D81050
diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
index d4bad34..c883a34 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkResult.h
@@ -74,7 +74,8 @@
   std::string Error;
   std::string Info;
   std::vector<uint8_t> AssembledSnippet;
-
+  // How to aggregate measurements.
+  enum ResultAggregationModeE { Min, Max, Mean, MinVariance };
   // Read functions.
   static Expected<InstructionBenchmark> readYaml(const LLVMState &State,
                                                  StringRef Filename);