Add a test

src/tests/JIT/Regression/JitBlue/Runtime_70466/Runtime_70466.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.CompilerServices;
+
+public class Runtime_70466
+{
+    public static int Main()
+    {
+        Problem(1, 0);
+        return 100;
+    }
+
+    // This method is carefully crafted such that we end up with an unused ARR_ADDR node by rationalization
+    // time, of which the child operand will need to be explicitly marked "unused value" for LIR purposes.
+    //
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Problem(byte b, int idx)
+    {
+        var a = new byte[] { b };
+        var a1 = new byte[] { b, b };
+        var a2 = new byte[] { b, b, b };
+
+        if (idx == 0)
+        {
+            if (a[idx] == 1)
+            {
+                Use(1);
+            }
+            JitUse(a1[idx] + a2[idx]);
+        }
+    }
+
+    public static void Use<T>(T arg) { }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void JitUse<T>(T arg) { }
+}

src/tests/JIT/Regression/JitBlue/Runtime_70466/Runtime_70466.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>