nir/lower_io: Add "explicit" IO lowering

nir/lower_io: Add "explicit" IO lowering

This new pass is for lowering explicitly laid out memory coming in from
SPIR-V or a similar source.  It's quite a bit more complicated than the
normal lower_io because we have to be able to handle matrices.  The
way the stride information is stored for matrices is awkward and dealing
with row-major matrices is especially painful.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>