nir/split_vars: Don't split arrays of cooperative matrix types

nir/split_vars: Don't split arrays of cooperative matrix types

glsl_type_is_vector_or_scalar would more accruately be called "can be an
r-value that isn't an array, structure, or matrix. This optimization
pass really shouldn't do anything to cooperative matrices. These
matrices will eventually be lowered to something else (dependent on the
backend), and that thing may (or may not) be handled by this or another
pass.

Fixes: 2d0f4f2c17b ("compiler/types: Add support for Cooperative Matrix types")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25871>
(cherry picked from commit 18d8a96a00a1666fcf202ae89aee6344d295687e)