xtensa: Small changes to 'return' insn pattern

xtensa: Small changes to 'return' insn pattern

This pattern is only emitted during function epilogue expansion (obviously
after register allocation), so putting reload_completed in the condition
is redundant.

This patch also changes the declaration of the return register (A0 address
register) required for normal function returns to properly defining the
EPILOGUE_USES macro, as is already done on other targets, rather than
placing '(use (reg:SI A0_REG))' RTX.

gcc/ChangeLog:

	* config/xtensa/xtensa.h (EPILOGUE_USES): New macro definition.
	* config/xtensa/xtensa.md (return):
	Remove '(use (reg:SI A0_REG))' from the template description, and
	reload_completed from the condition.
	(sibcall_epilogue): Remove emitting '(use (reg:SI A0_REG))'.