Verify that edge cases produce proper results, and some more.

[npitre@baylibre.com: avoid undefined shift value]
  Link: https://lkml.kernel.org/r/7rrs9pn1-n266-3013-9q6n-1osp8r8s0rrn@syhkavp.arg
Link: https://lkml.kernel.org/r/20240707190648.1982714-3-nico@fluxnic.netSigned-off-by: Nicolas Pitre <npitre@baylibre.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>