nir: Store the GL UBO buffer sizes in the shader info, use in nir_to_tgsi

This is useful information to have for a backend lowering GLSL on the frontend to eventually-GLSL on the backend after lowering to UBO intrinsics (such as virgl does through TGSI, see the ureg_DECL_constant2D() in st_translate_program()). Without this, you don't know how large to declare your UBO variables without duplicating the frontend's sizing logic.

(extracted from !8044 (merged) and tested on its own)