JIT: Kill assertions about struct local when skipping field stores in block morph

src/coreclr/jit/morphblock.cpp

@@ -242,8 +242,6 @@ void MorphInitBlockHelper::PropagateBlockAssertions()
 //
 void MorphInitBlockHelper::PropagateExpansionAssertions()
 {
-    // Consider doing this for FieldByField as well
-    //
     if (m_comp->optLocalAssertionProp && (m_transformationDecision == BlockTransformation::OneStoreBlock))
     {
         m_comp->fgAssertionGen(m_store);
@@ -400,6 +398,7 @@ void MorphInitBlockHelper::TryInitFieldByField()
         if (m_comp->fgGlobalMorph && m_dstLclNode->IsLastUse(i))
         {
             JITDUMP("Field-by-field init skipping write to dead field V%02u\n", fieldLclNum);
+            m_comp->fgKillDependentAssertionsSingle(m_dstLclNum DEBUGARG(m_store));
             continue;
         }
 
@@ -1242,6 +1241,7 @@ GenTree* MorphCopyBlockHelper::CopyFieldByField()
         {
             INDEBUG(unsigned dstFieldLclNum = m_comp->lvaGetDesc(m_dstLclNum)->lvFieldLclStart + i);
             JITDUMP("Field-by-field copy skipping write to dead field V%02u\n", dstFieldLclNum);
+            m_comp->fgKillDependentAssertionsSingle(m_dstLclNum DEBUGARG(m_store));
             continue;
         }
 

src/tests/JIT/Regression/JitBlue/Runtime_113658/Runtime_113658.cs

@@ -0,0 +1,34 @@
+// 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 Xunit;
+
+public static class Runtime_113658
+{
+    [Fact]
+    public static int TestEntryPoint()
+    {
+        FillStackWithGarbage();
+        long? nullable = FaultyDefaultNullable<long?>();
+        return (int)(100 + nullable.GetValueOrDefault());
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void FillStackWithGarbage()
+    {
+        stackalloc byte[256].Fill(0xcc);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    [SkipLocalsInit]
+    private static T FaultyDefaultNullable<T>()
+    {
+        // When T is a Nullable<T> (and only in that case), we support returning null
+        if (default(T) is null && typeof(T).IsValueType)
+            return default!;
+
+        throw new InvalidOperationException("Not nullable");
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_113658/Runtime_113658.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>

src/tests/JIT/Regression/JitBlue/Runtime_114358/Runtime_114358.cs

@@ -1,5 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
+
 // Found by Antigen
 // Reduced from 206.63 KB to 1.9 KB.
 

src/tests/JIT/Regression/JitBlue/Runtime_114978/Runtime_114978.cs

@@ -1,3 +1,6 @@
+// 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 v2.7 on 2025-04-23 21:44:41
 // Run on X64 Windows
 // Seed: 3309115426102150651-vectort,vector128,vector256,vector512,x86aes,x86avx,x86avx2,x86avx512bw,x86avx512bwvl,x86avx512cd,x86avx512cdvl,x86avx512dq,x86avx512dqvl,x86avx512f,x86avx512fvl,x86avx512fx64,x86avx512vbmi,x86avx512vbmivl,x86bmi1,x86bmi1x64,x86bmi2,x86bmi2x64,x86fma,x86lzcnt,x86lzcntx64,x86pclmulqdq,x86popcnt,x86popcntx64,x86sse,x86ssex64,x86sse2,x86sse2x64,x86sse3,x86sse41,x86sse41x64,x86sse42,x86sse42x64,x86ssse3,x86x86base