G1: Excessive is_obj_dead_cond calls in verification

While investigating JDK-8329314 it was found that G1 was slower than Parallel when allocating objects because of the extra verification.

This tiny tests takes 30s with G1:
public class Test {
    static int n = 847734685;

    public static void main(String[] args) {
         String[] strs = new String[n + 1];
    }
}

A large portion of that time is spent inside this assert:
    assert(!G1CollectedHeap::heap()->is_obj_dead_cond(_containing_obj, _vo), "Precondition");

which is called for every single oop in the _containing_obj. If I move this assert to where _containing_obj is initialized, the test now completes in 4-5 seconds.