This transitions vect_model_simple_cost to SLP only

This transitions vect_model_simple_cost to SLP only

As part of the vector cost API cleanup this transitions
vect_model_simple_cost to only record costs with SLP node.
For this to work the patch adds an overload to record_stmt_cost
only passing in the SLP node.

The vect_prologue_cost_for_slp adjustment is one spot that
needs an eye with regard to re-doing the whole thing.

	* tree-vectorizer.h (record_stmt_cost): Add overload with
	only SLP node and no vector type.
	* tree-vect-stmts.cc (record_stmt_cost): Use
	SLP_TREE_REPRESENTATIVE for stmt_vec_info.
	(vect_model_simple_cost): Do not get stmt_vec_info argument
	and adjust.
	(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.
	* tree-vect-slp.cc (vect_prologue_cost_for_slp): Use
	full-blown record_stmt_cost.