RFR: Fix handling of weak references during verification

[William Kemper]

This is fallout from our last big merge from upstream. When verifying reachable objects before evacuation during final mark, concurrent reference processing hasn't happened yet. For this reason, verification is required to ignore weak references which hold an unmarked referent (that would be cleared during reference processing). In our last merged, we inadvertently disabled the code to ignore weak references during certain verification phases.

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Commit messages:
 - Fix handling of weak references during verification

Changes: https://git.openjdk.java.net/shenandoah/pull/68/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=68&range=00
  Stats: 30 lines in 5 files changed: 0 ins; 22 del; 8 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/68.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/68/head:pull/68

PR: https://git.openjdk.java.net/shenandoah/pull/68