spirv: Don't remove variables used by resource indexing intrinsics

In Vulkan, for some variable modes, the generated NIR will have derefs
pointing to resource index intrinsics instead of the variable.  This
was letting nir_remove_dead_variables pass remove those variables,
which would lose information relevant for later passes after
spirv2nir.

Add a set to keep track of such variables and prevent them to be
removed when producing the NIR output.

Issue reported by Rhys.

Fixes: c4c9c780b1 ("spirv: Remove more dead variables")
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8706>

src/compiler/spirv/vtn_variables.c

@@ -237,6 +237,11 @@ vtn_variable_resource_index(struct vtn_builder *b, struct vtn_variable *var,
       desc_array_index = nir_imm_int(&b->nb, 0);
    }
 
+   if (b->vars_used_indirectly) {
+      vtn_assert(var->var);
+      _mesa_set_add(b->vars_used_indirectly, var->var);
+   }
+
    nir_intrinsic_instr *instr =
       nir_intrinsic_instr_create(b->nb.shader,
                                  nir_intrinsic_vulkan_resource_index);