LoongArch: Remove unused code.

LoongArch: Remove unused code.

gcc/ChangeLog:

	* config/loongarch/lasx.md: Remove unused code.
	* config/loongarch/loongarch-protos.h
	(loongarch_split_lsx_copy_d): Remove.
	(loongarch_split_lsx_insert_d): Ditto.
	(loongarch_split_lsx_fill_d): Ditto.
	* config/loongarch/loongarch.cc
	(loongarch_split_lsx_copy_d): Ditto.
	(loongarch_split_lsx_insert_d): Ditto.
	(loongarch_split_lsx_fill_d): Ditto.
	* config/loongarch/lsx.md (lsx_vpickve2gr_du): Remove splitter.
	(lsx_vpickve2gr_<lsxfmt_f>): Ditto.
	(abs<mode>2): Remove expander.
	(vabs<mode>2): Rename 2 abs<mode>2.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vector/lsx/lsx-abs.c: New test.