Those either depend on information filled by the NIR linking steps OR
are restricted by those:

- gl_nir_lower_samplers: depends on UniformStorage being set by the
  linker.

- brw_nir_lower_image_load_store: After 6981069f

 "i965: Ignore
  uniform storage for samplers or images, use binding info" we want
  this pass to happen after gl_nir_lower_samplers.

- gl_nir_lower_buffers: depends on UniformBlocks and
  SharedStorageBlocks being set by the linker.

For the regular GLSL code path, those datastructures are filled
earlier.  For NIR linking code path we need to generate the nir_shader
first then process it -- and currently the processing works with all
shaders together.  So move the passes out of brw_create_nir into its
own function, called by the brwProgramStringNotify and
brw_link_shader().

This patch prepares ground for ARB_gl_spirv, that will make use of NIR
linker.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>