RFR: 8372110: GenShen: Fix erroneous asssert

[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.

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

Commit messages:
 - Fix erroneous assert

Changes: https://git.openjdk.org/jdk/pull/28375/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28375&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372110
  Stats: 5 lines in 1 file changed: 2 ins; 1 del; 2 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