nir: Allow opt_peephole_select to work on empty blocks.

nir: Allow opt_peephole_select to work on empty blocks.

nir_opt_peephole_select has the job of removing IF statements with no side
effects.  However, if the IF statement's successor didn't have any
instructions in it, we were skipping it, which occurred in mupen64 on vc4
with glsl_to_nir enabled:

instructions in affected programs:     6134 -> 4120 (-32.83%)
total uniforms in shared programs: 38268 -> 38219 (-0.13%)

No changes on Haswell shader-db.

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