vect: Move VMAT_LOAD_STORE_LANES handlings from final loop nest

Like commit r14-3214 which moves the handlings on memory
access type VMAT_LOAD_STORE_LANES in vectorizable_load
final loop nest, this one is to deal with the function
vectorizable_store.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_store): Move the handlings on
	VMAT_LOAD_STORE_LANES in the final loop nest to its own loop,
	and update the final nest accordingly.