nir: Make nir_deref_instr_build/get_const_offset actually use size_align.

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.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>