LoongArch: Adjust the cost of ADDRESS_REG_REG.

LoongArch: Adjust the cost of ADDRESS_REG_REG.

After changing this cost from 1 to 3, the performance of spec2006
401 473 416 465 482 can be improved by about 2% on LA664.

Add option '-maddr-reg-reg-cost='.

gcc/ChangeLog:

	* config/loongarch/genopts/loongarch.opt.in: Add
	option '-maddr-reg-reg-cost='.
	* config/loongarch/loongarch-def.cc
	(loongarch_rtx_cost_data::loongarch_rtx_cost_data): Initialize
	addr_reg_reg_cost to 3.
	* config/loongarch/loongarch-opts.cc
	(loongarch_target_option_override): If '-maddr-reg-reg-cost='
	is not used, set it to the initial value.
	* config/loongarch/loongarch-tune.h
	(struct loongarch_rtx_cost_data): Add the member
	addr_reg_reg_cost and its assignment function to the structure
	loongarch_rtx_cost_data.
	* config/loongarch/loongarch.cc (loongarch_address_insns):
	Use la_addr_reg_reg_cost to set the cost of ADDRESS_REG_REG.
	* config/loongarch/loongarch.opt: Regenerate.
	* config/loongarch/loongarch.opt.urls: Regenerate.
	* doc/invoke.texi: Add description of '-maddr-reg-reg-cost='.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/const-double-zero-stx.c: Add
	'-maddr-reg-reg-cost=1'.
	* gcc.target/loongarch/stack-check-alloca-1.c: Likewise.