Adjust rtx_costs for h8300

Adjust rtx_costs for h8300

So the primary purpose of this patch is to make it easier to write tests for
removal of useless test/compare insns on the H8.

In simplest terms the costing model in the H8 port tends to encourage changing
something like:

  x = y + 4;
  if (x == 0)

into:

  x = y + 4;
  if (y == -4)

This is a marginal de-optimization on the H8.  So fixing it makes the code
ever-so-slightly better in isolation.   Fixing this also improves redundant
test/compare elimination and makes writing tests for redundant test/compare
elimination far easier.

gcc/
	* config/h8300/h8300.c (h8300_rtx_costs): Handle the various
	comparison rtx codes too.