vect: Remove some manual release in vectorizable_store

To avoid some duplicates in some follow-up patches on
function vectorizable_store, this patch is to adjust some
existing vec with auto_vec and remove some manual release
invocation.  Also refactor a bit and remove some uesless
codes.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_store): Remove vec oprnds,
	adjust vec result_chain, vec_oprnd with auto_vec, and adjust
	gvec_oprnds with auto_delete_vec.