LoongArch: NFC: Simplify logic of vec_perm{v32qi,v16hi}

LoongArch: NFC: Simplify logic of vec_perm{v32qi,v16hi}

De-duplicate the login by introducing gen_lasx_xvpermi_d (mode, ...)
with "@".  Also remove the merge_two label: we should really not (ab)use
goto when avoiding it is trivial.

Link: https://dl.acm.org/doi/10.5555/1241515.1241518
gcc/

	* config/loongarch/lasx.md (lasx_xvpermi_d): Add "@".
	* config/loongarch/loongarch.cc (loongarch_expand_vec_perm_1):
	Use gen_lasx_xvpermi_d instead of
	gen_lasx_xvpermi_d_{v32qi,v16hi} to deduplicate the logic.  Do
	structrual programming instead of goto and label.