JIT: (bug) AvxVnni.V512.MultiplyWideningAndAdd folded into BlendVariable loses the merge operand

[EgorBo]

Minimal repro

using System;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics;
using System.Runtime.Intrinsics.X86;

public static class Program
{
    [MethodImpl(MethodImplOptions.NoInlining)]
    public static Vector512<int> Test(Vector512<int> fallback, Vector512<int> addend,
                                      Vector512<byte> left, Vector512<sbyte> right,
                                      Vector512<int> a, Vector512<int> b)
    {
        Vector512<int> mask = Avx512F.CompareEqual(a, b);
        return Avx512F.BlendVariable(fallback, AvxVnni.V512.MultiplyWideningAndAdd(addend, left, right), mask);
    }

    public static void Main()
    {
        if (!AvxVnni.V512.IsSupported) { Console.WriteLine("unsupported"); return; }
        Console.WriteLine(Test(
            Vector512.Create(7),            // fallback
            Vector512.Create(5),            // addend (left*right == 0, so the product is 5)
            Vector512<byte>.Zero,
            Vector512<sbyte>.Zero,
            Vector512.Create(1),
            Vector512.Create(0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1)));
    }
}

Expected

<7, 5, 7, 5, 7, 5, 7, 5, 7, 5, 7, 5, 7, 5, 7, 5>

Actual

<5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5>

Notes

Repros under DOTNET_TieredCompilation=0; DOTNET_JitMinOpts=1 prints the expected value.
MultiplyWideningAndAddSaturate behaves the same way.

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[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• JIT: Invalid result with Avx2.BlendVariable #125431
• JIT: Debug/Release generate different results #114572

[EgorBo]

Affected SDK versions:

• 11.0.100-rc.2.26467.112