Fix memory leak in vectorizable_store

Fix memory leak in vectorizable_store

The following fixes a memory leak in vectorizable_store which happens
because the functions populating gvec_oprnds[i] will call .create ()
on the incoming vector, leaking what we've previously allocated.

	* tree-vect-stmts.cc (vectorizable_store): Do not allocate
	storage for gvec_oprnds elements.