nir: move lots of code from nir_lower_io.c into new nir_lower_explicit_io.c

nir_lower_io is just for regular inputs/outputs.

Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35760>
3 files changed