ira: Skip empty regclass when setting up reg class relations

ira.cc:setup_reg_class_relations sets up ira_reg_class_subset (among
other things). If reg class cl3 has no registers, then that empty set
is always hard_reg_set_subset_p of any other set, and this makes
ira_reg_class_subset[ALL_REGS][NO_REGS] equal to such a regclass,
rather than NO_REGS.

This breaks code (lra-constraints.cc:in_class_p/curr_insn_transform,
for e.g.) which uses NO_REGS to check for an empty regclass.

Why define an empty regclass? A regclass could be conditionally empty (via
TARGET_CONDITIONAL_REGISTER_USAGE) - for the avr target, ADDW_REGS and
NO_LD_REGS are empty for the avrtiny subarch, for example.

Fix by continuing the innermost loop if the corresponding reg class is empty.

gcc/ChangeLog:

	* ira.cc (setup_reg_class_relations): Continue
	if regclass cl3 is hard_reg_set_empty_p.