This is an umbrella RFE to track JIT specific follow-up work from JDK-8366705 / https://github.com/openjdk/valhalla/pull/1452

All items are marked with "// TODO 8366668" in the code:
- Re-enable IR verification in TestZGCBarrierElision.java, it was probably broken by https://github.com/openjdk/valhalla/pull/1452/commits/10457af257982948fc200f95ee6d4f5234bfb416

We should also evaluate if we can get rid of the _vm_type property in the C2 typesystem and if we should split ciObjArrayKlass into ciRefArrayKlass and ciFlatArrayKlass like the runtime now does.