RFR: 8370441: Remove unnecessary/confusing null check in Verifier::verify() [v2]

[Paul Hübner]

> [JDK-8059924](https://bugs.openjdk.org/browse/JDK-8059924) introduced a nullcheck for a klass' Java mirror which runs during verification. However, in the modern VM, during verification time, the mirror is guaranteed to exist making the check unnecessary and misleading when reading the code. While implementing [JDK-8366488](https://bugs.openjdk.org/browse/JDK-8366488) it caused for quite some confusion. With this PR it is my intention to make sure that nobody gets side-tracked by this impossible condition.
> 
> For an insight into previous discussion on this topic, please see the associated JBS issue.
> 
> Testing: tiers 1-5 on Linux x64, AArch64, macOS x64, AArch64, Windows x64.

Paul Hübner has updated the pull request incrementally with two additional commits since the last revision:

 - Keep the rest of the code.
 - Revert "Turn check into an assert."
   
   This reverts commit 78fa2983b532c4b390720316e79c975e61a4152a.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/29314/files
  - new: https://git.openjdk.org/jdk/pull/29314/files/78fa2983..1ecae607

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29314&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29314&range=00-01

  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29314.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29314/head:pull/29314

PR: https://git.openjdk.org/jdk/pull/29314