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

glsl_type_is_vector_or_scalar would more accurately 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: 2d0f4f2c ("compiler/types: Add support for Cooperative Matrix types")

I'm guessing that @bnieuwenhuizen and @daniel-schuermann didn't encounter problems here because they're lowering cooperative matrices earlier.