vect: Move VMAT_GATHER_SCATTER handlings from final loop nest

vect: Move VMAT_GATHER_SCATTER handlings from final loop nest

Like r14-3317 which moves the handlings on memory access
type VMAT_GATHER_SCATTER in vectorizable_load final loop
nest, this one is to deal with vectorizable_store side.

gcc/ChangeLog:

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