nir: Use the nir_builder _imm helpers in setting up deref offsets.

nir: Use the nir_builder _imm helpers in setting up deref offsets.

When looking at the dEQP nested_struct_array_dynamic_index_fragment code
after lowering, I was horrified at the amount of adding and multiplying by
0 we were doing.  The builder _imm helpers handle that for you so that the
following optimization passes have less work to do.  Plus, it's easier to
read.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>