gl_nir_lower_samplers_as_deref creates new top level sampler and image
uniforms which have been split from structure uniforms.  i965 assumed
that it could walk through gl_uniform_storage slots by starting at
var->data.location and walking forward based on a simple slot count.
This assumed that structure types were walked in a particular order.

With samplers and images split out of structures, it becomes impossible
to assign meaningful locations.  Consider:

   struct S {
      sampler2D a;
      sampler2D b;
   } s[2];

The gl_uniform_storage locations for these follow this map:

   0 => a[0], 1 => b[0], 2 => a[0], 3 => b[0].

But the new split variables look like:

   sampler2D lowered_a[2];
   sampler2D lowered_b[2];

and there is no way to know that there's effectively a stride to get to
the location for successive elements of a[] or b[].  So, working with
location becomes effectively impossible.

Ultimately, the point of looking at uniform storage was to pull out the
bindings from the opaque index fields.  gl_nir_lower_samplers_as_derefs
can obtain this information while doing the splitting, however, and sets
up var->data.binding to have the desired values.

We move gl_nir_lower_samplers before brw_nir_lower_image_load_store so
gl_nir_lower_samplers_as_derefs has the opportunity to set proper image
bindings.  Then, we make the uniform handling code skip sampler(-array)
variables, and handle image param setup based on var->data.binding.

Fixes Piglit tests/spec/glsl-1.10/execution/samplers/uniform-struct,
this time without regressing dEQP-GLES2.functional.uniform_api.random.3.

Fixes: f003859f

 nir: Make gl_nir_lower_samplers use gl_nir_lower_samplers_as_deref

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>