[CX] Long query compilation due to pushing down assign operators unnecessarily

One of the things that PushFieldAccessRule attempts to do is push an assign operator down as close as possible to the respective data scan operator. In the case where all the operators below the assign are other assign operators, pushing down the assign is not necessary since it's already close to the data scan operator. Pushing assign operators down blindly was shown to be problematic when the SELECT statement has a large number of fields causing the compilation of the query to take eternity.