vtn: Support cooperative matrices in OpConstantNull

Cooperative matrix initializers are a single scalar value that gets
broadcasted to the entire matrix.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12679
Fixes: b98f87612b ("spirv: Implement SPV_KHR_cooperative_matrix")
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33747>

src/compiler/spirv/spirv_to_nir.c

@@ -2256,6 +2256,7 @@ vtn_null_constant(struct vtn_builder *b, struct vtn_type *type)
    switch (type->base_type) {
    case vtn_base_type_scalar:
    case vtn_base_type_vector:
+   case vtn_base_type_cooperative_matrix:
       c->is_null_constant = true;
       /* Nothing to do here.  It's already initialized to zero */
       break;