vect: Also cost gconds for scalar [PR118211]

Currently we only cost gconds for the vector loop while we omit costing
them when analyzing the scalar loop; this unfairly penalizes the vector
loop in the case of loops with early exits.

This (together with the previous patches) enables us to vectorize
std::find with 64-bit element sizes.

gcc/ChangeLog:

	PR tree-optimization/118211
	PR tree-optimization/116126
	* tree-vect-loop.cc (vect_compute_single_scalar_iteration_cost):
	Don't skip over gconds.