vect: Avoid copying of uninitialized variable [PR118628]

vectorizable_{store,load} does roughly
      tree offvar;
      tree running_off;
      if (!costing_p)
        {
          ... initialize offvar ...
        }
      running_off = offvar;
      for (...)
        {
          if (costing_p)
            {
              ...
              continue;
            }
          ... use running_off ...
        }
so, it copies unconditionally sometimes uninitialized variable (but then
uses the copied variable only if it was set to something initialized).
Still, I think it is better to avoid copying around maybe uninitialized
vars.

2025-01-23  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/118628
	* tree-vect-stmts.cc (vectorizable_store, vectorizable_load):
	Initialize offvar to NULL_TREE.