[NDS32] Add relax optimization as new pass.

[NDS32] Add relax optimization as new pass.

gcc/
	* config.gcc (nds32): Add nds32-relax-opt.o into extra_objs.
	* config/nds32/constants.md (unspec_volatile_element): Add
	UNSPEC_VOLATILE_RELAX_GROUP.
	* config/nds32/nds32-relax-opt.c: New file.
	* config/nds32/nds32-predicates.c
	(nds32_symbol_load_store_p): New function.
	* config/nds32/nds32-protos.h
	(nds32_symbol_load_store_p): Declare function.
	(make_pass_nds32_relax_opt): Declare new rtl pass function.
	* config/nds32/nds32.c
	(nds32_register_pass): New function to register pass.
	(nds32_register_passes): New function to register passes.
	* config/nds32/nds32.md (relax_group): New pattern.
	* config/nds32/nds32.opt (mrelax-hint): New option.
	* config/nds32/t-nds32 (nds32-relax-opt.o): New dependency.

Co-Authored-By: Kito Cheng <kito.cheng@gmail.com>
Co-Authored-By: Kuan-Lin Chen <kuanlinchentw@gmail.com>
From-SVN: r258998