Enable vectorization for unknown tripcount in very cheap cost model but...

Enable vectorization for unknown tripcount in very cheap cost model but disable epilog vectorization.

gcc/ChangeLog:

	* tree-vect-loop.cc (vect_analyze_loop_costing): Enable
	vectorization for LOOP_VINFO_PEELING_FOR_NITER in very cheap
	cost model.
	(vect_analyze_loop): Disable epilogue vectorization in very
	cheap cost model.
	* doc/invoke.texi: Adjust documents for very-cheap cost model.