8372376: ZGC: Inaccurate verification of raw nulls in flip promoting pages

[fisk]

After JDK-8371200, flip promoted objects in ZGC get their age changed to old, before raw null pointers are pruned with promotion barriers. This is surprising to verification code that checks in the old generation mark end GC pause that no objects that are old have any raw null pointers. This PR relaxes the verification check to better understand flip promotions from the young generation.

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Issue

• JDK-8372376: ZGC: Inaccurate verification of raw nulls in flip promoting pages (Bug - P2)

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