tree-optimization/109573 - avoid ICEing on unexpected live def

tree-optimization/109573 - avoid ICEing on unexpected live def

The following relaxes the assert in vectorizable_live_operation
where we catch currently unhandled cases to also allow an
intermediate copy as it happens here but also relax the assert
to checking only.

	PR tree-optimization/109573
	* tree-vect-loop.cc (vectorizable_live_operation): Allow
	unhandled SSA copy as well.  Demote assert to checking only.

	* g++.dg/vect/pr109573.cc: New testcase.