ira.h (target_ira): Add x_ira_class_singleton.

ira.h (target_ira): Add x_ira_class_singleton.

gcc/
	* ira.h (target_ira): Add x_ira_class_singleton.
	(ira_class_singleton): New macro.
	* ira.c (setup_prohibited_class_mode_regs): Set up ira_class_singleton.
	* ira-build.c (update_conflict_hard_reg_costs): Use
	ira_class_singleton to check for classes with a single
	allocatable register.
	* ira-lives.c (ira_implicitly_set_insn_hard_regs): Likewise.
	(single_reg_class): Likewise.  When more than one class is specified,
	check whether they have the same singleton register.
	(process_single_reg_class_operands): Require single_reg_class
	to return NO_REGS or a class with a single allocatable register.
	Obtain that register from ira_class_singleton.

From-SVN: r191995