[24/46] Make stmt_info_for_cost use a stmt_vec_info

[24/46] Make stmt_info_for_cost use a stmt_vec_info

This patch makes stmt_info_for_cost carry a stmt_vec_info instead
of a gimple stmt.  The structure is internal to the vectoriser,
so targets aren't affected.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (stmt_info_for_cost::stmt): Replace with...
	(stmt_info_for_cost::stmt_info): ...this new field.
	(add_stmt_costs): Update accordingly.
	* tree-vect-loop.c (vect_compute_single_scalar_iteration_cost)
	(vect_get_known_peeling_cost): Likewise.
	(vect_estimate_min_profitable_iters): Likewise.
	* tree-vect-stmts.c (record_stmt_cost): Likewise.

From-SVN: r263139