spirv: Handle arbitrary bit sizes for deref array indices

spirv: Handle arbitrary bit sizes for deref array indices

We already had code in link_as_ssa to handle bit sizes; we just need to
use it.  While we're at it we clean up link_as_ssa a bit and add an
explicit bit_size parameter in preparation for a day when we have derefs
that aren't 32 bit.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>