LoongArch: Optimizations of vector construction.

LoongArch: Optimizations of vector construction.

gcc/ChangeLog:

	* config/loongarch/lasx.md (lasx_vecinit_merge_<LASX:mode>): New
	pattern for vector construction.
	(vec_set<mode>_internal): Ditto.
	(lasx_xvinsgr2vr_<mode256_i_half>_internal): Ditto.
	(lasx_xvilvl_<lasxfmt_f>_internal): Ditto.
	* config/loongarch/loongarch.cc (loongarch_expand_vector_init):
	Optimized the implementation of vector construction.
	(loongarch_expand_vector_init_same): New function.
	* config/loongarch/lsx.md (lsx_vilvl_<lsxfmt_f>_internal): New
	pattern for vector construction.
	(lsx_vreplvei_mirror_<lsxfmt_f>): New pattern for vector
	construction.
	(vec_concatv2df): Ditto.
	(vec_concatv4sf): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vector/lasx/lasx-vec-construct-opt.c: New test.
	* gcc.target/loongarch/vector/lsx/lsx-vec-construct-opt.c: New test.