nir: Make nir_deref_instr_build/get_const_offset actually use size_align.
I think this was copy-and-paste mistake -- nir_opt_large_constants was passing in glsl_get_natural_size_align_bytes() given brw_nir.c's arguments to the opt pass.
I wanted to reuse this function for handling constant offsets of arrays of images in V3D.