nir: save IO semantics in lowered IO intrinsics

nir: save IO semantics in lowered IO intrinsics

This enables drivers and utils to get all IO information from intrinsics,
so that they don't have to walk the complex types of NIR variables to find
out other information about IO intrinsics.

NIR in/out variables can be removed after nir_lower_io. We could remove
the variables in the pass, but for now I just decided to remove
the variables in radeonsi before shaders are returned to st/mesa.
(st/mesa just needs adjustments to work without NIR in/out variables)

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6442>