[release/8.0-staging] Ensure that the Create(Dot(...)) optimization doesn't kick in for Vector2 pre SSE4.1

src/coreclr/jit/morph.cpp

@@ -10724,6 +10724,16 @@ GenTree* Compiler::fgOptimizeHWIntrinsic(GenTreeHWIntrinsic* node)
                 break;
             }
 
+#if defined(TARGET_XARCH)
+            if ((node->GetSimdSize() == 8) && !compOpportunisticallyDependsOn(InstructionSet_SSE41))
+            {
+                // When SSE4.1 isn't supported then Vector2 only needs a single horizontal add
+                // which means the result isn't broadcast across the entire vector and we can't
+                // optimize
+                break;
+            }
+#endif // TARGET_XARCH
+
             GenTree* op1      = node->Op(1);
             GenTree* sqrt     = nullptr;
             GenTree* toScalar = nullptr;

src/tests/JIT/Regression/JitBlue/Runtime_96939/Runtime_96939.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;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using Xunit;
+
+public static class Runtime_96939
+{
+    [Fact]
+    [MethodImpl(MethodImplOptions.AggressiveOptimization)]
+    public static void Problem()
+    {
+        Assert.Equal(new Vector2(13), TestVector2(new Vector2(2, 3)));
+        Assert.Equal(new Vector3(29), TestVector3(new Vector3(2, 3, 4)));
+        Assert.Equal(new Vector4(54), TestVector4(new Vector4(2, 3, 4, 5)));
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static Vector2 TestVector2(Vector2 value)
+    {
+        return Vector2.Dot(value, value) * new Vector2(1, 1);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static Vector3 TestVector3(Vector3 value)
+    {
+        return Vector3.Dot(value, value) * new Vector3(1, 1, 1);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static Vector4 TestVector4(Vector4 value)
+    {
+        return Vector4.Dot(value, value) * new Vector4(1, 1, 1, 1);
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_96939/Runtime_96939.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <DebugType>None</DebugType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>