nir: Use the NIR_SRC_AS_ macro to define nir_src_as_deref
We have a macro for this now; no reason to hand-roll it for derefs. While we're here, move the NIR_DEFINE_CAST for derefs down to where all the other ones are.
We have a macro for this now; no reason to hand-roll it for derefs. While we're here, move the NIR_DEFINE_CAST for derefs down to where all the other ones are.