RISC-V: Add shorten_memrefs pass.

RISC-V: Add shorten_memrefs pass.

	gcc/
	* config.gcc:  Add riscv-shorten-memrefs.o to extra_objs for riscv.
	* config/riscv/riscv-passes.def: New file.
	* config/riscv/riscv-protos.h (make_pass_shorten_memrefs): Declare.
	* config/riscv/riscv-shorten-memrefs.c: New file.
	* config/riscv/riscv.c (tree-pass.h): New include.
	(riscv_compressed_reg_p): New Function
	(riscv_compressed_lw_offset_p): Likewise.
	(riscv_compressed_lw_address_p): Likewise.
	(riscv_shorten_lw_offset): Likewise.
	(riscv_legitimize_address): Attempt to convert base + large_offset
	to compressible new_base + small_offset.
	(riscv_address_cost): Make anticipated compressed load/stores
	cheaper for code size than uncompressed load/stores.
	(riscv_register_priority): Move compressed register check to
	riscv_compressed_reg_p.
	* config/riscv/riscv.h (C_S_BITS): Define.
	(CSW_MAX_OFFSET): Define.
	* config/riscv/riscv.opt (mshorten-memefs): New option.
	* config/riscv/t-riscv (riscv-shorten-memrefs.o): New rule.
	(PASSES_EXTRA): Add riscv-passes.def.
	* doc/invoke.texi: Document -mshorten-memrefs.

	* config/riscv/riscv.c (riscv_new_address_profitable_p): New function.
	(TARGET_NEW_ADDRESS_PROFITABLE_P): Define.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (TARGET_NEW_ADDRESS_PROFITABLE_P):  New hook.
	* sched-deps.c (attempt_change): Use old address if it is cheaper than
	new address.
	* target.def (new_address_profitable_p): New hook.
	* targhooks.c (default_new_address_profitable_p): New function.
	* targhooks.h (default_new_address_profitable_p): Declare.

	gcc/testsuite/
	* gcc.target/riscv/shorten-memrefs-1.c: New test.
	* gcc.target/riscv/shorten-memrefs-2.c: New test.
	* gcc.target/riscv/shorten-memrefs-3.c: New test.
	* gcc.target/riscv/shorten-memrefs-4.c: New test.
	* gcc.target/riscv/shorten-memrefs-5.c: New test.
	* gcc.target/riscv/shorten-memrefs-6.c: New test.
	* gcc.target/riscv/shorten-memrefs-7.c: New test.