glsl: tidy up uniform storage value count code in NIR linker

This makes the code cleaner and better reflects what the existing
glsl IR  linker does possibly fixing subtle bugs.

Reviewed-by: Alejandro PiƱeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4623>
1 file changed