JIT: Avoid clearing morphed flag in gtSetEvalOrder (dotnet#106286)

Call morphing calls `gtPrepareCost` as part of `EvalArgsToTemps` and
`SortArgs`. These functions are called when visiting the call node in
post-order, so clearing `GTF_DEBUG_NODE_MORPHED` here ends up clearing
it permanently from subtrees. Since post-order morphing for some nodes
can return child nodes, this allows for a scenario where post-order
morphing returns a descendant node that has had its
`GTF_DEBUG_NODE_MORPHED` flag cleared. Fix it by not clearing the flag.

Fix dotnet#106278

src/coreclr/jit/gentree.cpp

@@ -4904,11 +4904,6 @@ unsigned Compiler::gtSetEvalOrder(GenTree* tree)
         return gtSetEvalOrderMinOpts(tree);
     }
 
-#ifdef DEBUG
-    /* Clear the GTF_DEBUG_NODE_MORPHED flag as well */
-    tree->gtDebugFlags &= ~GTF_DEBUG_NODE_MORPHED;
-#endif
-
     /* Is this a FP value? */
 
     bool isflt = varTypeIsFloating(tree->TypeGet());

src/tests/JIT/Regression/JitBlue/Runtime_106278/Runtime_106278.cs

@@ -0,0 +1,31 @@
+// 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 v2.2 on 2024-08-12 14:54:43
+// Run on X64 Windows
+// Seed: 16070819126771301687-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 61.3 KiB to 0.3 KiB in 00:01:27
+// Hits JIT assert in Release:
+// Assertion failed '(tree->gtDebugFlags & GTF_DEBUG_NODE_MORPHED) && "ERROR: Did not morph this node!"' in 'Program:Main(Fuzzlyn.ExecutionServer.IRuntime)' during 'Morph - Global' (IL size 18; hash 0xade6b36b; FullOpts)
+// 
+//     File: D:\a\_work\1\s\src\coreclr\jit\morph.cpp Line: 12664
+// 
+
+using System;
+using Xunit;
+
+public class Runtime_106278
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        try
+        {
+            int[] vr0 = default(int[]);
+            double vr1 = -1216.4287239109472d % vr0[0];
+        }
+        catch (NullReferenceException)
+        {
+        }
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_106278/Runtime_106278.csproj

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