nir: Split nir_lower_io's input/output/atomic handling into helpers.

nir: Split nir_lower_io's input/output/atomic handling into helpers.

The original function was becoming a bit hard to read, with the details
of creating and filling out load/store/atomic atomics all in one
function.

This patch makes helpers for creating each type of intrinsic, and also
combines them with the *_op() helpers, as they're closely coupled and
not too large.

v2: Minor style nits from Jason.

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