RFR: 8333876: C2 SuperWord: regression after JDK-8325155: failed: internal connection

[Emanuel Peter]

Bug-fix for regression in [JDK-8325155](https://bugs.openjdk.org/browse/JDK-8325155) / https://github.com/openjdk/jdk/pull/18822.

I had added some assert code in that change, which did not previously exist. Now that assert code triggers on the regression test attached.

Solution: I converted the assert into a check.

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Commit messages:
 - JDK-8333876

Changes: https://git.openjdk.org/jdk/pull/19736/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19736&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333876
  Stats: 64 lines in 2 files changed: 60 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19736.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19736/head:pull/19736

PR: https://git.openjdk.org/jdk/pull/19736