[NDS32] Have shirnk-wrapping optimization to be performed on nds32 target.

[NDS32] Have shirnk-wrapping optimization to be performed on nds32 target.

gcc/
	* config/nds32/nds32-protos.h (nds32_can_use_return_insn): New.
	* config/nds32/nds32.md (unspec_volatile_func_return): Remove.
	(return_internal): New.
	(return): Define this named pattern.
	(simple_return): Define this named pattern.
	* config/nds32/nds32.c (nds32_expand_epilogue): Emit return_internal
	pattern instead of unspec_volatile_func_return.
	(nds32_expand_epilogue_v3pop): Likewise.
	(nds32_can_use_return_insn): New function.

From-SVN: r219711