anv/pipeline: Constant fold after apply_pipeline_layout

anv/pipeline: Constant fold after apply_pipeline_layout

Thanks to the new NIR load_descriptor intrinsic added by the UBO/SSBO
lowering series, we weren't getting UBO pushing because the UBO range
detection pass couldn't see the constants it needed.  This fixes that
problem with a quick round of constant folding.  Because we're folding
we no longer need to go out of our way to generate constants when we
lower the vulkan_resource_index intrinsic and we can make it a bit
simpler.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>