Fix typing of BlendVariableMask condition when optimizing TernaryLogic

TernaryLogic nodes normalize small types to large types. When optimizing
a TernaryLogic node to a BlendVariableMask, if we leave the normalized
type, then the Blend uses the wrong instruction.

To fix this, change the Blend node to use the mask simd base type.
We only do this for mask base types that are small, which is
unsatisfying: there exists codegen today for double->int casts,
for example, that uses a 'double' type mask but an 'int' sized
TernaryLogic node. This works because we end up only using lane '0', as
the vector is converted to scalar. I didn't find any other case like
this, so hopefully if the mask type is small we can safely use it.

Fixes #114572

Author: BruceForstall

src/coreclr/jit/lowerxarch.cpp

@@ -3875,6 +3875,28 @@ GenTree* Lowering::LowerHWIntrinsicTernaryLogic(GenTreeHWIntrinsic* node)
                 }
 
                 assert(varTypeIsMask(condition));
+
+                // The TernaryLogic node normalizes small SIMD base types on import. To optimize
+                // to BlendVariableMask, we need to "un-normalize". We no longer have the original
+                // base type, so we use the mask base type instead.
+                NamedIntrinsic intrinsicId = node->GetHWIntrinsicId();
+                assert(HWIntrinsicInfo::NeedsNormalizeSmallTypeToInt(intrinsicId));
+
+                // The condition mask element size is expected to be the same size as the TernaryLogic
+                // node element size, unless the condition element size is short, in which case the
+                // TernaryLogic node element size would have been normalized.
+                // However, there is code, such as for double->int conversions, that generates a 'double'
+                // base type mask for a TernaryLogic node with base type 'int'. That works because it only
+                // cares about the '0' elem, as the result will be cast to scalar.
+                var_types conditionBaseType = condition->AsHWIntrinsic()->GetSimdBaseType();
+                uint32_t  conditionElemSize = genTypeSize(conditionBaseType);
+                uint32_t  elemSize          = genTypeSize(simdBaseType);
+                if (varTypeIsShort(conditionBaseType) && (conditionElemSize < elemSize))
+                {
+                    CorInfoType simdBaseJitTypeCondition = condition->AsHWIntrinsic()->GetSimdBaseJitType();
+                    node->AsHWIntrinsic()->SetSimdBaseJitType(simdBaseJitTypeCondition);
+                }
+
                 node->ResetHWIntrinsicId(NI_EVEX_BlendVariableMask, comp, selectFalse, selectTrue, condition);
                 BlockRange().Remove(op4);
                 break;

src/tests/JIT/Regression/JitBlue/Runtime_114572/Runtime_114572.cs

@@ -0,0 +1,40 @@
+// 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 v2.5 on 2025-04-11 19:29:41
+// Run on X64 Windows
+// Seed: 557319528607462789-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 41.1 KiB to 0.7 KiB in 00:01:23
+// Debug: Outputs <0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1>
+// Release: Outputs <0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1>
+
+using System;
+using System.Numerics;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.X86;
+using Xunit;
+
+public class Runtime_114572
+{
+    public static Vector256<ushort> s_2;
+    public static ushort s_4;
+
+    [Fact]
+    public static void Problem()
+    {
+        if (Avx512F.VL.IsSupported)
+        {
+            var vr11 = Vector256.Create<ushort>(0);
+            var vr12 = Vector256.Create<ushort>(1);
+            var vr13 = (ushort)0;
+            var vr14 = Vector256.CreateScalar(vr13);
+            var vr15 = (ushort)1;
+            var vr16 = Vector256.CreateScalar(vr15);
+            var vr17 = Vector256.Create<ushort>(s_4);
+            var vr18 = Avx2.Max(vr16, vr17);
+            s_2 = Avx512F.VL.TernaryLogic(vr11, vr12, Avx512BW.VL.CompareGreaterThanOrEqual(vr14, vr18), 216);
+            System.Console.WriteLine(s_2);
+            Assert.Equal(Vector256.Create((ushort)0, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1, (ushort)1), s_2);
+        }
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_114572/Runtime_114572.csproj

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