Let's imagine that IO datatype is defined thus:

>{-# LANGUAGE GADTs #-}
>{-# LANGUAGE NoImplicitPrelude #-}

>import Prelude(Monad, Char)
>data IO a where
>    GetChar :: IO Char
>    Bind :: IO a -> (a -> IO b) -> IO b

>getChar = GetChar
>(>>=) = Bind

It is perfectly possible to construct IO actions as values of this data type and execute them by some function evalIO :: IO -> Prelude.IO with the obvious definition. Now the question arises: do you think
getChar >>= \x -> getChar would be optimized to getChar by compiler?