RFR: 8330165: C2: make superword consistently use PhaseIdealLoop::register_new_node()

[Roland Westrelin]

Another set of changes from 8275202. There are cases in superword
where new nodes are not assigned control. I believe they are harmless
currently because superword is the last pass of optimizations. I also
cleaned up the code so it always uses `register_new_node()`. There are
a couple places where `intcon()` should be used.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/18760/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330165
  Stats: 40 lines in 1 file changed: 1 ins; 17 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/18760.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18760/head:pull/18760

PR: https://git.openjdk.org/jdk/pull/18760