LoongArch:Implement avg and sad standard names.

gcc/ChangeLog:

	* config/loongarch/lasx.md
	(avg<mode>3_ceil): New patterns.
	(uavg<mode>3_ceil): Ditto.
	(avg<mode>3_floor): Ditto.
	(uavg<mode>3_floor): Ditto.
	(usadv32qi): Ditto.
	(ssadv32qi): Ditto.
	* config/loongarch/lsx.md
	(avg<mode>3_ceil): New patterns.
	(uavg<mode>3_ceil): Ditto.
	(avg<mode>3_floor): Ditto.
	(uavg<mode>3_floor): Ditto.
	(usadv16qi): Ditto.
	(ssadv16qi): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/avg-ceil-lasx.c: New test.
	* gcc.target/loongarch/avg-ceil-lsx.c: New test.
	* gcc.target/loongarch/avg-floor-lasx.c: New test.
	* gcc.target/loongarch/avg-floor-lsx.c: New test.
	* gcc.target/loongarch/sad-lasx.c: New test.
	* gcc.target/loongarch/sad-lsx.c: New test.