vect: Keep scalar costs around longer

vect: Keep scalar costs around longer

The scalar costs for a loop are fleeting, with only the final
single_scalar_iteration_cost being kept for later comparison.
This patch replaces single_scalar_iteration_cost with the cost
structure, so that (with later patches) it's possible for targets
to examine other target-specific cost properties as well.  This will
be done by passing the scalar costs to hooks where appropriate;
targets shouldn't try to read the information directly from
loop_vec_infos.

gcc/
	* tree-vectorizer.h (_loop_vec_info::scalar_costs): New member
	variable.
	(_loop_vec_info::single_scalar_iteration_cost): Delete.
	(LOOP_VINFO_SINGLE_SCALAR_ITERATION_COST): Delete.
	(vector_costs::total_cost): New function.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Update
	after above changes.
	(_loop_vec_info::~_loop_vec_info): Delete scalar_costs.
	(vect_compute_single_scalar_iteration_cost): Store the costs
	in loop_vinfo->scalar_costs.
	(vect_estimate_min_profitable_iters): Get the scalar cost from
	loop_vinfo->scalar_costs.