RFR (sh/jdk11): Relax Shenandoah/C2 verifier against JDK11 shortcomings

[Roman Kennke]

Nightlies still fail because our C2 verifier complains a bunch of things:
- It wants an LRB after a mirror-load. However, mirror-loads are not
guarded by a barrier in jdk11, and doesn't have to. Mirrors are
immutable so we don't care. Let's keep it simple and let the verifier
shut up ;-)

- A couple of intrinsics are a bit careless and don't check+cast their
inputs to not-null. Our verifier doesn't like. Let's special-case them
and silence the verifier. (We might want to fix the cases via jdk11u
properly.)

http://cr.openjdk.java.net/~rkennke/jdk11-c2-verifier-fixes/webrev.00/

Testing: checked the failing tests, hotspot_gc_shenandoah

Ok to push?

Roman