Naga `immediate_slots_required` calculation is incorrect

See https://github.com/gpuweb/cts/issues/4670.

We need to mark [accessibleSlots(T)](https://gpuweb.github.io/gpuweb/wgsl/#accessible-slots) as required, i.e. all slots of `var<immediate>` type without padding, instead of specific fields of `var<immediate>` type the expressions actually touch.