Emit a null check when loading valuetype fields

[bholmes]

We had a similar bug with stfld recently.
#1781
dotnet/runtime#82663

• Should this pull request have release notes?
  • Yes
  • No
• Do these changes need to be back ported?
  • Yes
  • No
• Do these changes need to be upstreamed to mono/mono or dotnet/runtime repositories?
  • Yes
  • No

Release notes

Fixed UUM-47983 @bholmes :
Mono: Fix runtime crash when accessing a struct field of a null object.

Backports

• 2022.3
• 2023.1
• 2023.2
• 2023.3

[bholmes]

The test I used to debug this

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public class Program
{
    public Program()
    {
    }

    static int Main(string[] args)
    {
        Foo defaultFoo = default;
        //GetBar().foo = defaultFoo;
        //defaultFoo = GetBar().foo;
        new Bar().foo =  GetBar().foo;
        return 0;
    }

    static Bar GetBar()
    {
        return null;
    }
}

public class Bar
{
    public Foo foo = default;
}

[StructLayout(LayoutKind.Explicit)]
public unsafe struct Foo
{
    [FieldOffset(0)] public object obj;
    [FieldOffset(8)] public fixed long Value[32];
}