nir/opt_cp: use nir_block_get_following_if()
nir/opt_cp: use nir_block_get_following_if()

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler