xtensa: Prepare the transition from Reload to LRA

xtensa: Prepare the transition from Reload to LRA

This patch provides the first step in the transition from Reload to LRA
in Xtensa.

gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h
	(xtensa_split1_finished_p, xtensa_split_DI_reg_imm): New prototypes.
	* config/xtensa/xtensa.cc
	(xtensa_split1_finished_p, xtensa_split_DI_reg_imm, xtensa_lra_p):
	New functions.
	(TARGET_LRA_P): Replace the dummy hook with xtensa_lra_p.
	(xt_true_regnum): Rework.
	* config/xtensa/xtensa.h (CALL_REALLY_USED_REGISTERS):
	Switch from CALL_USED_REGISTERS, and revise the comment.
	* config/xtensa/constraints.md (Y):
	Use !xtensa_split1_finished_p() instead of can_create_pseudo_p().
	* config/xtensa/predicates.md (move_operand): Ditto.
	* config/xtensa/xtensa.md: Add two new split patterns:
	  - splits DImode immediate load into two SImode ones
	  - puts out-of-constraint SImode constants into the constant pool
	* config/xtensa/xtensa.opt (-mlra): New target-specific option
	for testing purpose.