nir: Record non-vector/scalar varyings as unmovable when compacting

nir: Record non-vector/scalar varyings as unmovable when compacting

In some cases, we can end up with varying structs that aren't split to
their member variables.  nir_compact_varyings attempted to record these
as unmovable, so it would leave them be.  Unfortunately, it didn't do
it right for non-vector/scalar types.  It set the mask to:

   ((1 << (elements * dmul)) - 1) << var->data.location_frac

where elements is the number of vector elements.  For structures and
other non-vector/scalars, elements is 0...so the whole mask became 0.

This caused nir_compact_varyings to assign other varyings on top of
the structure varying's location (as it appeared to take up no space).

To combat this, we just set elements to 4 for non-vector/scalar types,
so that the entire slot gets marked as unmovable.

Fixes KHR-GL45.tessellation_shader.tessellation_control_to_tessellation_evaluation.gl_in on iris.

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