rs6000: Guard density_test only for vector version

This patch teaches rs6000_density_test to only care about the vector
version cost calculation and early return when calculating the single
scalar iteration cost.

Bootstrapped/regtested on powerpc64le-linux-gnu P9.

gcc/ChangeLog:

	* config/rs6000/rs6000.c (struct rs6000_cost_data): New member
	costing_for_scalar.
	(rs6000_density_test): Early return if costing_for_scalar is true.
	(rs6000_init_cost): Init costing_for_scalar of rs6000_cost_data.