LoongArch: Remove vec_concatz<mode> pattern.

It is incorrect to use vld/vori to implement the vec_concatz<mode> because when the LSX
instruction is used to update the value of the vector register, the upper 128 bits of
the vector register will not be zeroed.

gcc/ChangeLog:

	* config/loongarch/lasx.md (@vec_concatz<mode>): Remove this define_insn pattern.
	* config/loongarch/loongarch.cc (loongarch_expand_vector_group_init): Use vec_concat<mode>.