nir: Share get_io_offset handling in nir_lower_io.

nir: Share get_io_offset handling in nir_lower_io.

The load/store/atomic cases all duplicated the get_io_offset code, with
a few tiny differences: stores didn't bother checking for per-vertex
inputs, because they can't be stored to, and atomics didn't check at
all, since shared variables aren't per-vertex.

However, it's harmless to check, and allows us to share more code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>