Fixed IsTieredJitEnabled check in netcoreapp2.x.

Author: timcassell

src/BenchmarkDotNet/Portability/RuntimeInformation.cs

@@ -91,7 +91,7 @@ private static bool IsAotMethod()
                 // Disabled by default in netcoreapp2.X, check if it's enabled.
                 ? Environment.GetEnvironmentVariable("COMPlus_TieredCompilation") == "1"
                 || Environment.GetEnvironmentVariable("DOTNET_TieredCompilation") == "1"
-                || (AppContext.TryGetSwitch("System.Runtime.TieredCompilation", out bool isEnabled) && isEnabled)
+                || (AppContext.TryGetSwitch("System.Runtime.TieredCompilation", out bool isEnabled) && !isEnabled)
                 // Enabled by default in netcoreapp3.0+, check if it's disabled.
                 : Environment.GetEnvironmentVariable("COMPlus_TieredCompilation") != "0"
                 && Environment.GetEnvironmentVariable("DOTNET_TieredCompilation") != "0"