Fix and simplify vect_model_simple_cost

Fix and simplify vect_model_simple_cost

There's no longer any call without SLP node so we don't need dts.
I also figured based on the callers that some expect to cost
more than one stmt but still N times so this changes what we cost.

	* tree-vect-stmts.cc (vect_model_simple_cost): Removed code
	when no SLP node is passed, remove then unused dt and ndts
	parameters and fix the number of vector stmts calculation.
	(vectorizable_call): Adjust.
	(vectorizable_simd_clone_call): Likewise.
	(vectorizable_conversion): Likewise.
	(vectorizable_assignment): Likewise.
	(vectorizable_shift): Likewise.
	(vectorizable_operation): Likewise.
	(vectorizable_condition): Likewise.
	(vectorizable_comparison_1): Likewise.