Consider optimizing constrained calls to base methods on readonly struct in some cases

[jcouv]

Follow-up issue on comment #66189 (comment)

In the following test, we could avoid copying the instance of S1 as receiver for constrained call to object.ToString().
Aleksey's proposal: Perhaps we could optimize cases like this as long as consumer and the consumed type are in the same module.

        [Fact]
        public void InvokeOnThisBaseMethods()
        {
            var text = @"
class Program
{
    static void Main()
    {
        default(S1).Test();
    }
    readonly struct S1
    {
        public void Test()
        {
            System.Console.Write(this.GetType());
            System.Console.Write(ToString());
        }
    }
}
";

            var comp = CompileAndVerify(text, parseOptions: TestOptions.Regular, verify: Verification.Passes, expectedOutput: @"Program+S1Program+S1");

            comp.VerifyIL("Program.S1.Test()", @"
{
  // Code size       47 (0x2f)
  .maxstack  1
  .locals init (Program.S1 V_0)
  IL_0000:  ldarg.0
  IL_0001:  ldobj      ""Program.S1""
  IL_0006:  box        ""Program.S1""
  IL_000b:  call       ""System.Type object.GetType()""
  IL_0010:  call       ""void System.Console.Write(object)""
  IL_0015:  ldarg.0
  IL_0016:  ldobj      ""Program.S1""
  IL_001b:  stloc.0
  IL_001c:  ldloca.s   V_0
  IL_001e:  constrained. ""Program.S1""
  IL_0024:  callvirt   ""string object.ToString()""
  IL_0029:  call       ""void System.Console.Write(string)""
  IL_002e:  ret
}");
        }

If we can avoid this copy, we would instead have the following IL (like before):

            comp.VerifyIL("Program.S1.Test()", @"
{
  // Code size       39 (0x27)
  .maxstack  1
  IL_0000:  ldarg.0
  IL_0001:  ldobj      ""Program.S1""
  IL_0006:  box        ""Program.S1""
  IL_000b:  call       ""System.Type object.GetType()""
  IL_0010:  call       ""void System.Console.Write(object)""
  IL_0015:  ldarg.0
  IL_0016:  constrained. ""Program.S1""
  IL_001c:  callvirt   ""string object.ToString()""
  IL_0021:  call       ""void System.Console.Write(string)""
  IL_0026:  ret
}");

Relates to PR #66189 which fixed some correctness issues with such constrained calls.