Inspection on too restrictive SPECIALIZE

[chshersh]

If you have a function of type:

foo :: MonadIO m => m a

then the following specializations are either useless or too restrictive:

{-# SPECIALIZE foo :: MonadIO m => m () #-}
{-# SPECIALIZE foo :: IO () #-}

The only correct one is:

{-# SPECIALIZE foo :: IO a #-}

But since GHC doesn't warn on such cases, it's useful for Stan to provide such type of analysis.