RFR: 8372110: GenShen: Fix erroneous assert [v2]

[Kelvin Nilsen]

> Code recently integrated (https://github.com/openjdk/jdk/pull/27353) had an erroneous (meaningless) assert.  Closer examination of intended behavior revealed that the fix involves replacing the assert with a new run-time check, as implemented here.

Kelvin Nilsen 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 two additional commits since the last revision:

 - Merge remote-tracking branch 'jdk/master' into fix-finish-block-start-assert
 - Fix erroneous assert

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28375/files
  - new: https://git.openjdk.org/jdk/pull/28375/files/86d92901..591f09e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28375&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28375&range=00-01

  Stats: 5609 lines in 146 files changed: 3405 ins; 891 del; 1313 mod
  Patch: https://git.openjdk.org/jdk/pull/28375.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28375/head:pull/28375

PR: https://git.openjdk.org/jdk/pull/28375