Simplify intereaved store vectorization processing

When doing interleaving we perform code generation when visiting the
last store of a chain.  We keep track of this via DR_GROUP_STORE_COUNT,
the following localizes this to the caller of vectorizable_store,
also avoing redundant non-processing of the other stores.

	* tree-vect-stmts.cc (vectorizable_store): Do not bump
	DR_GROUP_STORE_COUNT here.  Remove early out.
	(vect_transform_stmt): Only call vectorizable_store on
	the last element of an interleaving chain.