RFR: Assert code roots are to-space ptrs when scanning/updating other roots

[Roman Kennke]

Instead of not scanning code roots at all when we don't have to, we
could check that all the oops in them actually point to to-space
objects as we're expecting. Implemented like this:

http://cr.openjdk.java.net/~rkennke/assertcoderoots/webrev.00/

Tested using hotspot_gc_shenandoah and some spot testing with specjvm.
Should not affect release builds.

Ok to push?

Roman