RFR: 8237543: Shenandoah: More asserts around code roots

[Roman Kennke]

We are still observing occasional corrupted code roots in Traversal GC.
The assert always happens in code roots, and always at init-traversal.
There are two ways this seems likely to happen: either when new code is
generated, or during the previous GC cycle. We should plant some
verifications there to ensure we fail earlier.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8237543
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8237543/webrev/

Testing: hotspot_gc_shenandoah (fastdebug+release) ok

Can I please get a review?

Thanks,
Roman