RFR: 8338717: GenShen: Fix up some assertions [v2]

[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:**
> - [x] GHA
> - [x] jtreg:tier1
> - [ ] Codepipeline (investigating what appears to be an unrelated failure I encountered in testing)
> - [ ] SPECjbb w/GenShen (in progress)

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into review_assert
 - Unref variable deleted so as to eliminate -werror on Windows.
 - Fix up some assertions etc. based on review feedback from Shipilev.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/478/files
  - new: https://git.openjdk.org/shenandoah/pull/478/files/b95c26df..21061519

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=478&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=478&range=00-01

  Stats: 119 lines in 8 files changed: 55 ins; 42 del; 22 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