RFR: 8275298: Remove unnecessary weak_oops_do call in adjust weak roots phase

[Albert Mingkun Yang]

This PR consists of two logical steps in two commits:

1. change `weak_oops_do` to `verify_no_references_recorded` to check discovered lists (DL) are indeed empty during adjust-weak-roots phase
2. Move the assertion to the end of ref-processing, since elements on DL are not really related to "weak roots"

Test: tier1-6

-------------

Commit messages:
 - move assert
 - ref-dl-roots

Changes: https://git.openjdk.java.net/jdk/pull/5957/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5957&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275298
  Stats: 37 lines in 9 files changed: 4 ins; 30 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5957.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5957/head:pull/5957

PR: https://git.openjdk.java.net/jdk/pull/5957