Fix for #21011: propagate GTF_DONT_CSE on comma returns

When a method returns a multi-reg struct, we set GTF_DONT_CSE on return local: https://github.com/dotnet/coreclr/blob/497419bf8f19c649d821295da7e225e55581cce9/src/jit/importer.cpp#L8783

Setting GTF_DONT_CSE blocks assertion propagation here:
https://github.com/dotnet/coreclr/blob/9d49bf1ec6f102b89e5c2885e8f9d3d77f2ec144/src/jit/assertionprop.cpp#L2845-L2848

In the test we have a synchronized method so we change the return node to
return a comma that include a call to HELPER.CORINFO_HELP_MON_EXIT.
If the rightmost comma expression doesn't have GTF_DONT_CSE,
assertion propagation is not blocked and we end up with this tree:

```
               [000040] -----+------                 /--*  CNS_INT   struct 0
               [000043] --C-G+------              /--*  COMMA     struct
               [000036] --C-G+------              |  \--*  CALL help void   HELPER.CORINFO_HELP_MON_EXIT
               [000032] L----+------ arg1 in x1   |     +--*  ADDR      long
               [000031] ----G+-N----              |     |  \--*  LCL_VAR   ubyte (AX) V03 tmp1
               [000033] -----+------ arg0 in x0   |     \--*  LCL_VAR   ref    V00 this
               [000041] -AC-G+------              *  COMMA     struct
               [000006] -----+-N----              |  /--*  LCL_VAR   struct V01 loc0
               [000039] -A---+------              \--*  ASG       struct (copy)
               [000037] D----+-N----                 \--*  LCL_VAR   struct V05 tmp3
```

Downstream phases can't handle struct zero return expressed as
```
               [000040] -----+------                 /--*  CNS_INT   struct 0
```

The fix is to propagate GTF_DONT_CSE to the rightmost comma expression
to block bad assertion propagation.

Fixes #21011.

2 files changed, 57 insertions, 0 deletions

diff --git a/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.cs b/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.cs
new file mode 100644
index 0000000000..9f485c3193
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.cs
@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Collections.Generic;
+
+public class Test
+{
+    public static int Main()
+    {
+        Test test = new Test();
+        test.GetPair();
+        return 100;
+    }
+
+    [MethodImpl(MethodImplOptions.Synchronized | MethodImplOptions.NoInlining)]
+    internal KeyValuePair<uint, float>? GetPair()
+    {
+        KeyValuePair<uint,float>? result = new KeyValuePair<uint,float>?();
+        return result;
+    }
+}
diff --git a/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.csproj b/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.csproj
new file mode 100644
index 0000000000..42f8a01f39
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/Github_21011/Github_21011.csproj
@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{2649FAFE-07BF-4F93-8120-BA9A69285ABB}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <ProjectTypeGuids>{786C830F-07A1-408B-BD7F-6EE04809D6DB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
+  </PropertyGroup>
+  <!-- Default configurations to help VS understand the configurations -->
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' "></PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' "></PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <CodeAnalysisDependentAssemblyPaths Condition=" '$(VS100COMNTOOLS)' != '' " Include="$(VS100COMNTOOLS)..\IDE\PrivateAssemblies">
+      <Visible>False</Visible>
+    </CodeAnalysisDependentAssemblyPaths>
+  </ItemGroup>
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
+  <PropertyGroup Condition=" '$(MsBuildProjectDirOverride)' != '' "></PropertyGroup>
+</Project>
\ No newline at end of file