RFR: 8338717: GenShen: Fix up some assertions

[Y. Srinivas Ramakrishna]

In https://github.com/openjdk/jdk/pull/20395 Aleksey provided some suggestions for improving various assertions. This ticket gathers together several of those changes.

**Testing:**
- [ ] GHA
- [ ] jtreg:tier1
- [ ] Codepipeline
- [ ] SPECjbb w/GenShen

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Commit messages:
 - Unref variable deleted so as to eliminate -werror on Windows.
 - Fix up some assertions etc. based on review feedback from Shipilev.

Changes: https://git.openjdk.org/shenandoah/pull/478/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=478&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338717
  Stats: 74 lines in 9 files changed: 49 ins; 8 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah/pull/478.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/478/head:pull/478

PR: https://git.openjdk.org/shenandoah/pull/478