[release/7.0] Ensure that the shuffle zero mask copies all bits

src/coreclr/jit/gentree.cpp

@@ -21830,13 +21830,11 @@ GenTree* Compiler::gtNewSimdShuffleNode(var_types   type,
 
     if (needsZero)
     {
-        assert(!compIsaSupportedDebugOnly(InstructionSet_SSSE3));
+        assert((simdSize == 32) || !compIsaSupportedDebugOnly(InstructionSet_SSSE3));
 
         op2                          = gtNewVconNode(type, simdBaseJitType);
-        op2->AsVecCon()->gtSimd16Val = mskCns.v128[0];
-
-        GenTree* zero = gtNewZeroConNode(type, simdBaseJitType);
-        retNode       = gtNewSimdCndSelNode(type, op2, retNode, zero, simdBaseJitType, simdSize, isSimdAsHWIntrinsic);
+        op2->AsVecCon()->gtSimd32Val = mskCns;
+        retNode = gtNewSimdBinOpNode(GT_AND, type, op2, retNode, simdBaseJitType, simdSize, isSimdAsHWIntrinsic);
     }
 
     return retNode;

src/tests/JIT/Regression/JitBlue/GitHub_85129/GitHub_85129.cs

@@ -0,0 +1,32 @@
+// 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;
+using System.Runtime.Intrinsics;
+using Xunit;
+
+public class Program_85129
+{
+    public static int Main()
+    {
+        Vector256<int> v256Shuffle = Vector256.Create(100, 101, 102, 103, 104, 105, 106, 107);
+        Vector256<int> v256ShuffleExpectedResult = Vector256.Create(107, 105, 0, 101, 106, 104, 0, 100);
+        Vector256<int> v256ShuffleActualResult = Vector256Shuffle(v256Shuffle);
+        if(v256ShuffleExpectedResult != v256ShuffleActualResult)
+        {
+            return 1;
+        }
+
+        return 100;
+    }
+
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static Vector256<int> Vector256Shuffle(Vector256<int> v1)
+    {
+        return Vector256.Shuffle(v1, Vector256.Create(7, 5, 132, 1, 6, 4, -3, 0));
+    }
+}

src/tests/JIT/Regression/JitBlue/GitHub_85129/GitHub_85129.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>PdbOnly</DebugType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>