nir: add nir_lower_uniforms_to_ubo pass

nir: add nir_lower_uniforms_to_ubo pass

This is a further lowering of default-block uniform loads that transforms
load_uniform intrinsics into load_ubo intrinsics. This simplifies the rest
of the backend.

v2: transform from load_uniform instead of straight from variables

Reviewed-by: Eric Anholt <eric@anholt.net>