RFR: 8291781: assert(!is_visited) failed: visit only once with -XX:+SuperWordRTDepCheck

[Fei Gao]

`-XX:+SuperWordRTDepCheck` is a develop flag and misses proper implementation. But when enabled, it could change code path, resulting in the failures in [JDK-8291781](https://bugs.openjdk.org/browse/JDK-8291781) and [JDK-8291881](https://bugs.openjdk.org/browse/JDK-8291881). As @vnkozlov suggested in [JDK-8291781](https://bugs.openjdk.org/browse/JDK-8291781), the small patch converts the flag to pure debug code to avoid effect on code generation.

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Commit messages:
 - 8291781: assert(!is_visited) failed: visit only once with -XX:+SuperWordRTDepCheck

Changes: https://git.openjdk.org/jdk/pull/10868/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291781
  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10868.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10868/head:pull/10868

PR: https://git.openjdk.org/jdk/pull/10868