JIT: also check gc-ness of layout for GT_BLK during escape analysis (#115844)

Otherwise we may mistakenly build a connection to the unknown source and
try retyping a non-gc struct.

Fixes #115831.

Author: AndyAyersMS

src/coreclr/jit/objectalloc.cpp

@@ -1906,6 +1906,19 @@ void ObjectAllocator::AnalyzeParentStack(ArrayStack<GenTree*>* parentStack, unsi
                     break;
                 }
 
+                // For structs we need to check the layout as well
+                //
+                if (parent->OperIs(GT_BLK))
+                {
+                    ClassLayout* const layout = parent->AsBlk()->GetLayout();
+
+                    if (!layout->HasGCPtr())
+                    {
+                        canLclVarEscapeViaParentStack = false;
+                        break;
+                    }
+                }
+
                 GenTree* const addr = parent->AsIndir()->Addr();
 
                 // For loads from local structs we may be tracking the underlying fields.

src/tests/JIT/opt/ObjectStackAllocation/Runtime_115831.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Generated by Fuzzlyn v3.0 on 2025-05-20 21:40:51
+// Run on X64 Windows
+// Seed: 11262416476118799467-vectort,vector128,vector256,x86aes,x86avx,x86avx2,x86avx512bw,x86avx512bwvl,x86avx512cd,x86avx512cdvl,x86avx512dq,x86avx512dqvl,x86avx512f,x86avx512fvl,x86avx512fx64,x86bmi1,x86bmi1x64,x86bmi2,x86bmi2x64,x86fma,x86lzcnt,x86lzcntx64,x86pclmulqdq,x86popcnt,x86popcntx64,x86sse,x86ssex64,x86sse2,x86sse2x64,x86sse3,x86sse41,x86sse41x64,x86sse42,x86sse42x64,x86ssse3,x86x86base
+// Reduced from 52.0 KiB to 0.5 KiB in 00:03:45
+// Hits JIT assert in Release:
+// Assertion failed 'newType == TYP_I_IMPL' in 'Program:Main(Fuzzlyn.ExecutionServer.IRuntime)' during 'Allocate Objects' (IL size 47; hash 0xade6b36b; FullOpts)
+// 
+//     File: D:\a\_work\1\s\src\coreclr\jit\objectalloc.cpp Line: 2427
+// 
+using System;
+using System.Numerics;
+using Xunit;
+
+public class C1
+{
+}
+
+public struct S2
+{
+    public Vector<ushort> F0;
+}
+
+public struct S3
+{
+    public S2 F3;
+    public C1 F5;
+}
+
+public class Runtime_115831
+{
+    public static S3 s_1;
+    [Fact]
+    public static void Problem()
+    {
+        S3 vr0 = s_1;
+        S2 vr1 = vr0.F3;
+        vr0.F5 = new C1();
+        System.Console.WriteLine(vr1.F0);
+    }
+}

src/tests/JIT/opt/ObjectStackAllocation/Runtime_115831.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>