LoongArch: testsuite: Fix gcc.dg/vect/vect-reduc-mul_{1, 2}.c FAIL.

LoongArch: testsuite: Fix gcc.dg/vect/vect-reduc-mul_{1, 2}.c FAIL.

This FAIL was introduced from r14-6908. The reason is that when merging
constant vector permutation implementations, the 128-bit matching situation
was not fully considered. In fact, the expansion of 128-bit vectors after
merging only supports value-based 4 elements set shuffle, so this time is a
complete implementation of the entire 128-bit vector constant permutation,
and some structural adjustments have also been made to the code.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_expand_vselect): Adjust.
	(loongarch_expand_vselect_vconcat): Ditto.
	(loongarch_try_expand_lsx_vshuf_const): New, use vshuf to implement
	all 128-bit constant permutation situations.
	(loongarch_expand_lsx_shuffle): Adjust and rename function name.
	(loongarch_is_imm_set_shuffle): Renamed function name.
	(loongarch_expand_vec_perm_even_odd): Function forward declaration.
	(loongarch_expand_vec_perm_even_odd_1): Add implement for 128-bit
	extract-even and extract-odd permutations.
	(loongarch_is_odd_extraction): Delete.
	(loongarch_is_even_extraction): Ditto.
	(loongarch_expand_vec_perm_const): Adjust.