Fix evaluation order of assignments to fields.

[mattmccutchen-microsoft]

Example program that demonstrates the incorrect translation by BCT:

using System;
using System.Diagnostics.Contracts;

public class Program
{
    class Cell
    {
        public int data;
    }
    static int last = 0;
    static Cell LHS(Cell cell)
    {
        last = 1;
        return cell;
    }
    static int RHS()
    {
        last = 2;
        return 42;
    }
    public static void Main()
    {
        Cell c = new Cell();
        LHS(c).data = RHS();
        Console.WriteLine(last);
        Contract.Assert(last == 2);
    }
}