nir: Avoid splitting compact arrays into per-element variables.

nir: Avoid splitting compact arrays into per-element variables.

Compact arrays are used for special variables like clip and cull
distances, or tessellation levels.  Drivers using compact arrays
assume that these values will always be actual arrays.  We don't
want to turn a float[1] gl_CullDistance into a single float; that
would confuse drivers.

Today, i965 uses compact arrays, and Gallium drivers use
nir_lower_io_arrays_to_elements, so we haven't had any overlap
that would demonstrate the issue.  Iris will use both.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>