Remove CreateFalse node during lowering, if MaskToVector is getting removed

src/coreclr/jit/lowerarmarch.cpp

@@ -4158,14 +4158,7 @@ GenTree* Lowering::LowerHWIntrinsicCndSel(GenTreeHWIntrinsic* cndSelNode)
                 // CndSel(mask, embedded(trueValOp2), op3)
                 //
                 cndSelNode->Op(2) = nestedCndSel->Op(2);
-                if (nestedOp3->IsMaskZero())
-                {
-                    BlockRange().Remove(nestedOp3);
-                }
-                else
-                {
-                    nestedOp3->SetUnusedValue();
-                }
+                nestedOp3->SetUnusedValue();
 
                 BlockRange().Remove(nestedOp1);
                 BlockRange().Remove(nestedCndSel);
@@ -4202,14 +4195,7 @@ GenTree* Lowering::LowerHWIntrinsicCndSel(GenTreeHWIntrinsic* cndSelNode)
                 op2->SetUnusedValue();
             }
 
-            if (op3->IsMaskZero())
-            {
-                BlockRange().Remove(op3);
-            }
-            else
-            {
-                op3->SetUnusedValue();
-            }
+            op3->SetUnusedValue();
             op1->SetUnusedValue();
 
             GenTree* next = cndSelNode->gtNext;

src/tests/JIT/Regression/JitBlue/Runtime_113338/Runtime_113338.cs

@@ -0,0 +1,35 @@
+// 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 2025-03-30 17:25:31
+// Run on Arm64 Windows
+// Seed: 6164687101299398040-vectort,vector64,vector128,armadvsimd,armadvsimdarm64,armaes,armarmbase,armarmbasearm64,armcrc32,armcrc32arm64,armdp,armrdm,armrdmarm64,armsha1,armsha256,armsve
+// Reduced from 35.7 KiB to 0.7 KiB in 00:00:46
+// Hits JIT assert in Release:
+// Assertion failed 'node->IsUnusedValue() && "found an unmarked unused value"' in 'Program:Main(Fuzzlyn.ExecutionServer.IRuntime)' during 'Lowering nodeinfo' (IL size 95; hash 0xade6b36b; FullOpts)
+// 
+//     File: D:\a\_work\1\s\src\coreclr\jit\lir.cpp Line: 1706
+// 
+using System;
+using System.Numerics;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.Arm;
+
+public class Program
+{
+    public static uint s_1;
+    public static void Main()
+    {
+        var vr7 = Sve.CreateTrueMaskInt32();
+        var vr8 = Vector.Create<int>(0);
+        var vr9 = Sve.CreateFalseMaskInt32();
+        var vr10 = Sve.ConditionalSelect(vr7, vr8, vr9);
+        var vr11 = Vector128.CreateScalar(12616752854119074364UL).AsVector();
+        var vr12 = (int)Sve.SaturatingIncrementByActiveElementCount(0, vr11);
+        var vr13 = Vector128.CreateScalar(vr12).AsVector();
+        if (Sve.TestLastTrue(vr10, vr13))
+        {
+            System.Console.WriteLine(s_1);
+        }
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_113338/Runtime_113338.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <!-- SYSLIB50003: System.Runtime.Intrinsics.Arm.Sve is experimental -->
+   <NoWarn>$(NoWarn);SYSLIB5003</NoWarn>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>