RFR: 8330165: C2: make superword consistently use PhaseIdealLoop::register_new_node() [v2]

[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.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18760/files
  - new: https://git.openjdk.org/jdk/pull/18760/files/f85804cc..fabd851a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18760&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18760&range=00-01

  Stats: 20 lines in 5 files changed: 3 ins; 0 del; 17 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