Silent bad codegen when returning a modified struct

[briansull]

Repro:

Compile MutateStructArg.cs below:

.\Tools\dotnetcli\dotnet.exe Tools\csc.exe /o+ /out:Program.exe MutateStructArg.cs /r:.\bin\tests\Windows_NT.x64.Checked\Tests\Core_Root\System.Private.CoreLib.dll /r:.\bin\tests\Windows_NT.x64.Checked\Tests\Core_Root\System.Console.dll
/r:.\bin\tests\Windows_NT.x64.Checked\Tests\Core_Root\System.Runtime.dll

MutateStructArg.cs

using System;
class MutateStructArg
{
   public struct P
   {
      public string S;
      public int    X;
   }

   public static int Main()
   {
      P l1 = new P();
      l1.S = "Hello World";
      l1.X = 42;
      P l2 = foo(l1);
      Console.WriteLine(l2.S); // Print modified value "Goodbye World"
      if ((l2.S == "Goodbye World") && (l2.X == 100))
      {
         return 100;   // success
      }
      else
      {
         Console.WriteLine("**** Test FAILED ***");
         return 1;  // failure
      }
   }

   public static P foo(P a)
   {
      Console.WriteLine(a.S);   // Print the incoming value "Hello World"
      a.S = "Goodbye World";    // Mutate the incoming value
      a.X = 100;
      return a;                 // Copy the modified value to the return value 
   }
}

Run the Repro:
.\bin\tests\Windows_NT.x64.Checked\Tests\Core_Root\CoreRun.exe MutateStructArg.exe
Hello World
Hello World
**** Test FAILED ***

[briansull]

This is a recent regression

[briansull]

@JosephTremoulet Can you take a look at this one

[briansull]

Note that this test failure corresponds to desktop test failure:

runbaseline_simple2_jit64-gc-misc-simple2.txt
jit\jit64\gc\misc\simple2.exe

[briansull]

@dotnet/jit-contrib FYI