JIT: don't dead store OSR exposed locals (#89867)

Backport of #89743 to 7.0.

The local var ref count for OSR locals can be misleading, since some
of the appearances may be in the "tier0" parts of the methods and
won't be visible once we trim the OSR method down to just the part
we're going to compile.

Fix by giving OSR-exposed locals an implicit ref count.

Fixes #89666.

Author: AndyAyersMS

src/coreclr/jit/lclvars.cpp

@@ -356,6 +356,12 @@ void Compiler::lvaInitTypeRef()
                 {
                     lvaSetVarAddrExposed(lclNum DEBUGARG(AddressExposedReason::OSR_EXPOSED));
                 }
+
+                // Ensure that ref counts for exposed OSR locals take into account
+                // that some of the refs might be in the Tier0 parts of the method
+                // that get trimmed away.
+                //
+                varDsc->lvImplicitlyReferenced = 1;
             }
         }
     }

src/tests/JIT/opt/OSR/Runtime_89666.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+public class Runtime_89666
+{
+    public static int Main()
+    {
+        byte[] a1 = new byte[100_000];
+        byte[] a2 = new byte[100_000];
+
+        Problem(a1.Length, a1);
+        Problem(a2.Length, a2);
+
+        for (int i = 0; i < a1.Length; i++)
+        {
+            if (a1[i] != a2[i])
+            {
+                Console.WriteLine($"Found diff at {i}: {a1[i]} != {a2[i]}");
+                return -1;
+            }
+        }
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Problem(int n, byte[] a)
+    {
+        Random random = new Random(1);
+        int value = 0;
+        Span<byte> span = MemoryMarshal.AsBytes(MemoryMarshal.CreateSpan(ref value, 1));
+        for (int i = 0; i < n; i++)
+        {
+            // This write must be kept in the OSR method
+            value = random.Next();
+            Write(span, i, a);
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Write(Span<byte> s, int i, byte[] a)
+    {
+        a[i] = s[0];
+    }
+}

src/tests/JIT/opt/OSR/Runtime_89666.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <DebugType />
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredPGO" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_QuickJitForLoops" Value="1" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TC_OnStackReplacement" Value="1" />
+  </ItemGroup>
+</Project>