JIT: Fix reordering with 'backfilled' arguments on x86 (#71940)

In a previous change I made the assumption that all register args come
before stack args on x86. This assumption is not true.

Fix #71939

src/coreclr/jit/morph.cpp

@@ -820,9 +820,8 @@ void CallArgs::ArgsComplete(Compiler* comp, GenTreeCall* call)
                 if (prevArg.AbiInfo.GetRegNum() == REG_STK)
                 {
                     // All stack args are already evaluated and placed in order
-                    // in this case; we only need to check this for register
-                    // args.
-                    break;
+                    // in this case.
+                    continue;
                 }
 #endif
 
@@ -902,7 +901,7 @@ void CallArgs::ArgsComplete(Compiler* comp, GenTreeCall* call)
                 {
                     // All stack args are already evaluated and placed in order
                     // in this case.
-                    break;
+                    continue;
                 }
 #endif
 
@@ -965,7 +964,7 @@ void CallArgs::ArgsComplete(Compiler* comp, GenTreeCall* call)
                         {
                             // All stack args are already evaluated and placed in order
                             // in this case.
-                            break;
+                            continue;
                         }
 #endif
                         // Invariant here is that all nodes that were not

src/tests/JIT/Regression/JitBlue/Runtime_71939/Runtime_71939.cs

@@ -0,0 +1,42 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Adapted from:
+// Generated by Fuzzlyn v1.5 on 2022-07-10 17:11:48
+// Run on X86 Windows
+// Seed: 2002556551479521504
+// Reduced from 184.6 KiB to 0.9 KiB in 00:03:45
+// Debug: Outputs 0
+// Release: Outputs -10
+
+using System;
+using System.Runtime.CompilerServices;
+
+public class Runtime_71939
+{
+    public static int Main()
+    {
+        int v3 = 1;
+        return Check(0, -v3, Mutate(ref v3));
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static int Mutate(ref int arg)
+    {
+        arg = 10;
+        return 0;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static int Check(ulong arg0, int arg1, int arg2)
+    {
+        if (arg1 != -1)
+        {
+            Console.WriteLine("FAIL: mutated arg is {0}", arg1);
+            return -1;
+        }
+
+        Console.WriteLine("PASS: mutated arg is {0}", arg1);
+        return 100;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_71939/Runtime_71939.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>