nir/lower_locals_to_regs: Avoid an iadd(0, x) for .base_offset == 0.
Noticed while tracing NIR-to-TGSI shaders in dEQP-GLES3.functional.shaders.indexing.varying_array.vec4_dynamic_loop_write_static_loop_read
I think the logic is clearer this way around, too.