Loom: Avoid putting stale object pointers in oops

Generational ZGC has extra verification code for oops, which trigger asserts when it finds stale oops. We have cleaned away some usages of stale oops in the upstream repository (openjdk/jdk), but there are still a couple left in the Loom code. I propose that we rewrite the code, to pave the way for Generational ZGC.