rs6000: Move rs6000_vect_nonmem into target cost_data

This patch is to move rs6000_vect_nonmem (target cost_data
related information) into target cost_data struct.
As Richi pointed out, we can gather data from add_stmt_cost
invocations.  This is one pre-step to centralize target
cost_data related stuffs.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (rs6000_vect_nonmem): Renamed to
	vect_nonmem and moved into...
	(struct rs6000_cost_data): ...here.
	(rs6000_init_cost): Use vect_nonmem of cost_data instead.
	(rs6000_add_stmt_cost): Likewise.
	(rs6000_finish_cost): Likewise.