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

 [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