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.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
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