vect: Factor out the handling on scatter store having gs_info.decl

Similar to the existing function vect_build_gather_load_calls,
this patch is to factor out the handling on scatter store
having gs_info.decl to vect_build_scatter_store_calls which
is a new function.  It also does some minor refactoring like
moving some variables' declarations close to their uses and
restrict the scope for some of them etc.

It's a pre-patch for upcoming vectorizable_store re-structuring
for costing.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vect_build_scatter_store_calls): New, factor
	out from ...
	(vectorizable_store): ... here.