Improve SImode shifts for H8

Improve SImode shifts for H8

Similar to the H8/300H patch, this improves SImode shifts for the H8/S.
It's not as big a win on the H8/S since we can shift two positions at a
time.  But that also means that we can handle more residuals with minimal
ode growth after a special shift-by-16 or shift-by-24 sequence.

I think there's more to do here, but this seemed like as good a checkpoint
as any.  Tested without regressions.

gcc/
	* config/h8300/h8300.c (shift_alg_si): Avoid loops for most SImode
	shifts on the H8/S.
	(h8300_option_override): Use loops on H8/S more often when optimizing
	for size.
	(get_shift_alg): Handle new "special" cases on H8/S.  Simplify
	accordingly.  Handle various arithmetic right shifts with special
	sequences that we couldn't handle before.